Thousands of Venezuelans angry over widespread shortages, increasing poverty and soaring inflation are descending on the capital, Caracas, to demand a referendum to recall President Nicolas Maduro in a massive protest that opposition leaders are calling the takeover of Caracas.

The demonstrators have arrived in the capital by bus, by car and even by foot from all corners of the country, including from the southern state of Amazonas bordering Brazil, the eastern states on the Atlantic Coast and the western Andes.

Leaders of the opposition — an ideologically diverse coalition of parties that aim to replace the current government — say they expect more than a million people to attend, with several smaller rallies taking place elsewhere around the country, making today's protest the largest opposition-led demonstration in two years. In 2014 opposition supporters demanded the resignation of the socialist Maduro in weeks of demonstrations that ended with 43 people killed, dozens injured and several opposition leaders detained.

This demonstration could turn violent, with pro-government national guard officers and militias roaming the streets, tear gas in hand, according to local reports and protesters in contact with ABC News. The country's national guard has already blocked access to several demonstration routes in the city; a prominent government party leader, Diosdado Cabello, said Wednesday that government sympathizers would block entry to the city.

"Not only will [protesters] not come in," he said at a rally, "but they also won't go out."

Opposition organizers have called for a peaceful gathering, but Maduro and other government leaders are calling today's demonstration an attempted coup orchestrated by the U.S. He blames an "economic war" planned by the opposition and the U.S. government as the cause for many of the country's woes.

He accuses the opposition of trying to oust Venezuela's legitimate government. He defeated opposition leader Henrique Capriles in the 2013 presidential election, called after the death of President Hugo Chavez.

Recently Maduro declared that if anyone attempted to stage a coup against his government, he would make Turkey's recent purge of adversaries look "like child's play."

Just this week, a former opposition mayor detained after the 2014 protests was transferred from house arrest to a military prison. Two opposition youth leaders arrested after the 2014 clashes were transferred from one prison to another, according to family members. A current opposition mayor in the central state of Aragua was also detained, as were 11 opposition youth leaders in the eastern state of Anzoategui.

Foreign press — including journalists from The Miami Herald, Al Jazeera and NPR — were not allowed to enter the country this week.

Today's protest comes as the oil-rich but cash-strapped country struggles in the face of low oil prices. Venezuelans have been standing in lines for up to 35 hours a week to buy scarce food, and hospitals have been turning away patients for lack of resources. The World Bank estimates that the country's 2016 gross domestic product will decline by more than 10 percent. The International Monetary Fund says inflation in the country will reach 500 percent this year and triple that next year. Venezuelans are experiencing shortages for about 80 percent of different kinds of goods, according to a local polling firm.

The demonstration today is the latest push for a presidential recall referendum, with the opposition facing a deadline to stage a vote to oust the president.

Earlier this year, opposition politicians collected nearly 2 million signatures in four days in favor of the referendum — more than the number required to start the recall process in the country of 30 million. When Venezuela's electoral authority required signatures to be revalidated, people lined up to sign again, officially starting the recall process. Now referendum supporters are asking the electoral authority to initiate the next step, allowing them to collect 20 percent of eligible signatures favoring a recall.

But the call for a referendum has faced backlash from government supporters. In July a group of opposition leaders touring the country to boost support for the recall were met by angry protesters. While the opposition coalition won control of the Venezuelan Congress in the country's most recent election in December, loyalty to the late Chavez is still palpable among different sectors around the country, even as Maduro's approval rating is in the single digits.

Today protesters are asking for a date to submit the new signatures. After that, the country's electoral authority would have to set a date for the referendum. If a vote is not held this year, even if the pro-referendum block wins, the presidency would stay in the hands of pro-Maduro forces, since Venezuela's Constitution stipulates that the vice president, Aristobulo Isturiz, would assume power.

Maduro has said no votes will be held until 2017.

If a recall vote takes place, it would be the second since the socialist party, PSUV, led by Chavez, came to power in 1998. The first presidential recall referendum was held in 2004 to oust Chavez but failed to gather enough support.