(CNN) -- U.S. Sen. John McCain took in the anti-government protest scene in Kiev's Maidan square Saturday, snapping pictures with his cell phone as he met with opposition leaders.
He called it an "incredible display of patriotism" in a tweet that includes an aerial photo of the square. McCain also said he was with the younger brother of heavyweight boxing champion Vitali Klitschko, who is one of the opposition leaders.
The Republican senator has been critical of Ukrainian authorities' use of force against protesters.
"If Ukraine's government thinks that brute force and the politics of fear can see it through the current crisis, it is woefully mistaken," McCain said in a statement last week.
Ukrainian prosecutors are investigating some top government officials regarding the beating of students at anti-government protests two weeks ago.
The subjects of the criminal investigation include Ukrainian Deputy Secretary for National Security Vladimir Sivkovich, Kiev Police Chief Valery Koryak and Kiev Mayor Oleksandr Popov. A fourth person also is under investigation.
The case will be put before a judge on Monday, who could order the those being investigated to house arrest, said Valery Vilkova, spokeswoman for the general prosecutor.
The tumult in Ukraine goes to the heart of its future ties with Russia and the rest of Europe.
Protests have unfolded in Ukraine since November 21 after President Viktor Yanukovych changed his stance on a trade pact with the European Union that had been years in the making.
He refused to sign the agreement, which would have strengthened cooperation with the European Union. Instead, he opted for closer ties with neighboring Russia.
Hundreds of thousands of Ukrainians have taken to the streets of Kiev to demand that the EU deal be signed.
The demonstrations are the biggest the country has seen since the Orange Revolution overturned the results of the country's 2004 presidential election. Protesters have created camps and barricades throughout the city center, and have set up makeshift soup kitchens, said CNN iReporter Harvey Carrol.
The epicenter of the protest has been in the city's central Maidan Square, where protesters have created a giant "revolutionary" Christmas tree decorated with flags and a giant poster of jailed former Prime Minister Yulia Tymoshenko.
On Saturday, a pro-government rally was held at nearby European Square, according to the state news agency Ukrinform. Kiev police estimated about 57,000 people participated in the rally. Ukrainian Prime Minister Mykola Azarov spoke at the gathering.