At least 50,000 demonstrators have gathered in a central square in Kiev, reviving Ukraine's slowing protest movement just as opposition leaders called for international sanctions against the government.

The mass rally in Independence Square came as opposition leader Arseniy Yatsenyuk announced that he would be sending an envoy to a US Senate hearing on Wednesday to press Washington to impose economic sanctions against "the gang now violating the law and the constitution", referring to the government.

Sunday's demonstration was the largest so far this year against President Victor Yanukovych, who drew the ire of thousands of Ukrainians in November after he decided to reject a trade deal with the European Union in favour of closer cooperation with Russia.

"We will continue to fight," Vitaly Klitschko, an opposition leader and former heavyweight boxing champion, told the protesters.

"And in order to be heard, we are going to organise a national strike - first a short one, as a warning, and then a full-scale one that lasts a long time," Klitschko said.

Dwindling numbers

The protest came after Ukrainian riot police used force to disperse protesters outside a courthouse in the capital, injuring at least ten people.

Yuriy Lutsenko, the former interior minister and opposition leader, was reportedly beaten by police on Friday. "He received about 10 blows to the head," Lutsenko's spokeswoman said.

Protests in Ukraine attracted as many as 800,000 people at their height last year.

But the protests quieted over New Year and the Ukrainian Orthodox Christmas, celebrated on January 7.

The opposition has repeated calls for early presidential elections, which are not due until 2015.

These demands have been ignored by the government, which was bolstered in mid-December after Yanukovych agreed to a $15bn bailout package with Russia.

The next opposition protest in Kiev is scheduled for January 19.