pains to urge protesters not to resort to action that would provide a pretext for a crackdown.
When clashes broke out about 500 metres (yards) from Independence Square, Klitschko went to the scene and sought to persuade protesters to refrain from attacking police.
"Stop your actions," he called to groups of young people - some of them masked. "We are a peaceful protest." Protesters sprayed a powder fire extinguisher at police, catching Klitschko whose face was covered in white.
As police later appeared to be readying to take a tougher line against protesters, he tweeted: "Viktor Yanukovich, do not go down the same road as (late Romanian dictator Nicolae) Ceausescu and (late Libyan leader Muammar) Gaddafi. Stop conducting war on the citizens of Ukraine."
Arseny Yatsenyuk, another opposition leader, told the crowds on Independence Square, "Our victory is not in using physical violence but in moral and spiritual strength."
Although setting up an alternative power structure may not be realistic, Sunday's turnout suggested it could also be difficult for the authorities to try to solve the crisis by use of force despite the court ban and the new laws.
Another opposition leader, far-right-nationalist Oleh Tyahnibok, dismissed the laws as unconstitutional as he spoke from the tribune on Kiev's main Independence Square.
"So we have a right not to carry them out and we will sabotage them," he said.
Yanukovich triggered the pro-Europe rallies when he did an about-turn last November and ditched a free trade deal with the European Union in favour of closer economic ties with Russia.
Russia has since thrown Ukraine a $15 billion lifeline in credits as well as a softer deal for purchases of strategic supplies of natural gas.