Vitali Klitschko, the world boxing champion and opposition leader, urged police to avoid escalating tensions with the protesters, who have taken to the streets in anger at the government's refusal to sign an integration deal with the European Union.
"The demonstration is peaceful. These people could be your mothers, your sisters, your daughters. They came here because they want the country to be a better place," he told officers at the barricades.
Reports emerged that armed riot police had raided the headquarters of the Fatherland party led by Yulia Tymoshenko, the jailed opposition leader, removing computer servers.
The brewing confrontation came after Viktor Yanukovich, the embattled Ukrainian president, agreed to talks with the pro-European opposition, announcing that he would hold a meeting today with Leonid Kravchuk, Leonid Kuchma and Viktor Yushchenko, his three former post-independence predecessors who have offered to broker dialogue.
Baroness Ashton, the EU foreign policy chief, was to fly to Kiev in an attempt to find a solution.