"I am the definite winner of this presidential election," Mousavi said at a news conference in Tehran. Mousavi's supporters say the former prime minister was taking the "upper hand" and was expected to win two-thirds of the vote.
Mousavi's election would be a huge upset victory. Just weeks ago, Admadinejad was expected to cruise to reelection.
While Mousavi declared victory, Iran's state-run media declared Ahmadinejad the winner. According to Iran's state-run TV, Ahmadinejad had garnered 68.8 percent and Mousavi 28.8 percent of the 10 million votes counted thus far.
Mousavi and other candidates have alleged voting fraud and while there is no concrete data backing Mousavi's claims of a victory, the candidate and his aides made it clear that if official numbers do not show him to be victorious, they will fight them.
Official numbers are expected to be released on Saturday.
Iranians flocked to the polls to vote for their next president in an election of unprecedented interest.
Many locations extended voting hours to accommodate the throngs of people. Iran's news agency reported that more than 70 percent of Iranians who are eligible to vote participated in Friday's election.
Long lines wrapped around nearly every polling center ABC News visited, revealing a deep interest in this election, which has pitted current Ahmadinejad against popular reformist candidate Mousavi.
The month leading up to Iran's 10th presidential election was filled with a rare show of democratic energy -- heated public debates, mass demonstrations and rowdy rallies in streets around the country.
President Obama said today that he is "excited" about the "robust debate" taking place in Iran, and that it shows change is possible.
"Ultimately the election is for the Iranians to decide, but just as has been true in Lebanon, what can be true in Iran as well is that you're seeing people looking at new possibilities, and whoever ends up winning the election in Iran, the fact that there's been a robust debate hopefully will help advance our ability to engage them in new ways," Obama said.
In his speech to the Muslim world last week, the U.S. president said he wants to open dialog with Iran and move forward away from decades of animosity "on the basis of mutual respect." But he also chided the country's leadership for its nuclear weapons ambitions.
"American policy, with respect to Iran and its nuclear program, is not dependent on which administration is governing Iran," U.N. Ambassador Susan said today. "Our view is that ... the Islamic Republic of Iran ought to not pursue its nuclear program, its nuclear-weapons program, and that will not change, depending on the outcome of the election."
Rice said the United States was not endorsing any specific candidate.
"I think we were certainly heartened by the large turnout in Lebanon and what appears to be a large turnout in Iran," she said.
The draw of the election -- which the world is watching closely -- is so powerful that even some expatriates have returned to the country to cast their ballots.
Ibrahim, an Iranian-American who declined to give his full name, flew from California to Tehran to vote for a change in foreign policy.
"We would like to have common goals between United States and European countries," he told ABC News.
For Iran's young people, it's about a wide variety of issues, everything from high inflation and rising unemployment to the social and religious climate to better relations with the West. The involvement of Iran's young -- nearly one-third of the voters are estimated to be 30 years old or younger -- means that stakes are high for the presidential candidates. In the past, much of Iran's young population has boycotted past elections, but for the first time, many young Iranians believe they can make a difference.
One example of their involvement: When the text messaging network went down on Wednesday, many young voters were dismayed they couldn't get their message across to their friends. The system remained down today. Even the popular social networking site Facebook was shut down temporarily, causing angst among a few.
"In the past, they used to say we block sites that have immoral information. Later, this included politics and Web sites that wrote against the government and these were filtered. But now they seem to be overdoing it by filtering Web sites that are not in line with their political party," said one Mousavi supporter, Barzegar. "However, this will not have a noticeable effect on keeping people from knowing their favorite candidates."
Some say such moves indicate that the government is blocking out the opposition's voice.
"The TV has not even informed people clearly about the exact time of Mr. Mousavi's speech on Channel One [a local TV station]," said Masoumeh Salmani, a Mousavi supporter, earlier this week.
Ahmadinejad vs. Mousavi: High-Stakes Vote
A slew of issues facing the next president -- ranging from the development of Iran's nuclear program to relations with Israel and the United States -- could mean a whole new foreign policy direction for the country.
The two leading candidates have very different political visions.
Ahmadinejad, fiery in his anti-American foreign policy, nuclear agenda and support for strict Islamic practices, faces a tough battle against Mousavi, a moderate who has called for more social freedom and more dialog with the West.
Iran's elections, powered by high energy among the country's young and old alike, have very much resembled Western-style campaigns with televised debates -- a first for Iran -- and internet campaigns. Mousavi even campaigned with his wife, another first for Iran, which became an Islamic republic 30 years ago.
Mousavi has been campaigning on a message of change and hope -- not unlike the message of President Obama's in 2008 -- which has energized young men and women in Iran's cities.
"The feature of a progressed nation is that, while it preserves its origins, it always has a look toward the future. We do not want a person who is backward, petrified and inclined toward the past," Mousavi said at a rally.
He has also called for a more open dialog with the United States.
"In foreign policy, our nation's dignity has been harmed. Our country has been degraded. Development inside the country has faced problems. There has been increasing tension with other countries," Mousavi said in the debate with Ahmadinejad. "I welcome the entire nation to support me and vote for me to change this situation."
The former prime minister has also captured the support of former president Mohammed Khatami, who is also calling for a more reformist agenda.
"We must guard this great achievement [of reform] and try not to let bandits and ill-minded to steal it from us," Khatami said a rally for Mousavi.
Mousavi's wife, Zahra Rahnavard, has also emerged as a key critic of Ahmadinejad's policies, a rare sight for the wife of a presidential candidate.
"Today, we feel that an atmosphere of freedom of speech, press and thought, which we are all interested in and have confidence in, is absent. We feel that we do not possess an independent and great economy because of the wrong policies and adventurous behavior at a national and international level, and because of unilateral decisions without consultation with experts," Rahnavard said at a political rally. "Now is the time we feel that we must be present on the scene."
"The Iranian mentality is changing. The generation is changing. Because of this, 'change' and 'yes we can' is the slogan of all of the reformist candidates," said political analyst Saeed Laylez.
"At present, our national security is at the highest possible level. There is no threat anymore and in the political field we have had great achievements. ... Iran is the superpower of the region," Ahmadinejad said in a TV interview. "That I am giving letters to Mr. Obama, it is not addressed to Obama, it is rather addressed to all humanity. ... It shapes world public opinion in favor of us. Today, fortunately, world public opinion is in favor of us."
Presidential candidate and liberal cleric Mehdi Karoubi also called for better relations with the United States.
"We don't want confrontation, or looking for an enemy," he told ABC News. "We want interaction with the world."
Karoubi added that if there is an opportunity for reconciliation with the United States, with which Iran's relations have been strained for more than three decades, he would grab it.
If Ahmadinejad garners more than 50 percent of today's votes, he wins automatically. Otherwise, there will be a one-week runoff between the two highest vote-getters.