(Credit: /Dmitry Lovetsky/AP Photo)

Sen. John McCain, R-Ariz., addressed a massive crowd of Ukrainian anti-government protestors in Kiev today, pledging his support for their cause.

"The free world is with you, America is with you, I am with you," he said.

For weeks now, protestors have occupied the main square in the capital Kiev, at times clashing with riot police, after Ukraine's President Viktor Yanukovich abandoned plans for a trade association with the European Union. He opted instead to join a customs union with Russia.

The decision, and the protests, are not just about a trade agreement. They are about the future of Ukraine and whether it will be more closely aligned with Europe and the West or Moscow.

In the square on Sunday, the mood was defiant. The crowd swelled to around 200,000 people despite the bitter cold. They sang and listened to speeches, urging the government to step down and embrace a future with Europe.

McCain arrived in Ukraine on Saturday with fellow Sen. Christopher Murphy, D-Conn. He met with Ukrainian opposition leaders, including Vitali Klitschko, the reigning WBA heavyweight boxing champion and now one of Ukraine's most popular politicians.

In a recent interview with ABC News, Klitschko said the fight for political change will likely take a long time.

"It's not one round. It's not two rounds. It may be 12," he said, using a boxing analogy.

Asked if he thinks the opposition will win, he replied: "Yes of course. If you don't believe in yourself you never win."

The protests have gotten Hollywood's attention. Actress Hayden Panetierre, the star of the ABC series "Nashville" who is engaged to Klitschko's brother, recently addressed the crowd.

"You have a right to a democracy," she told them.

George Clooney sent his own video, wishing them "good luck."

"When you look to the West, know that we are looking back at you with great admiration," he said.

Tonight, the European Union broke off talks with Ukraine, saying the president has shown no signs of signing a trade deal.

The protestors in the square say they aren't leaving until he does.