"We are issuing an ultimatum to authorities -- if they do not immediately announce the complete and unconditional rehabilitation (of protesters) in some 2,000 cases, we will retake city hall," Andriy Illenko, a lawmaker from the nationalist opposition Svoboda (Freedom) party, told protesters.
"This ultimatum expires in several hours," he warned, as around 150 protesters gathered in front of the building, some wearing helmets and army jackets, hitting metal batons on the ground.
The opposition had occupied Kiev's city hall since December as part of anti-government unrest that has raged in Ukraine for close to three months after President Viktor Yanukovych ditched a key EU trade pact in favour of closer ties with Russia.
But in a conciliatory gesture early today, up to 700 protesters evacuated the building in response to a last-minute concession by authorities who on Friday released all protesters detained in unrest.
The interior ministry said occupied government buildings in four other cities in Ukraine had also been vacated.
But Kiev's iconic Independence Square remained under opposition control -- the sprawling anti-government enclave protected on all sides against riot police by manned barricades -- as did several other buildings in the capital.
The evacuations were one of the conditions set in an amnesty law approved by Yanukovych early this month, promising to heed opposition demands to release detainees and drop charges against them.
Some protesters -- including at least one disabled man and a retiree -- have been slapped with charges carrying a sentence of up to 15 years in jail.
On Friday, just as negotiations between the warring parties had wound down, authorities unexpectedly announced they had freed all 234 people detained in the protest movement and promised to drop charges too if some occupied parts of Kiev were vacated.
In Brussels today, EU foreign policy chief Catherine Ashton hailed the release of the detainees but called on Kiev to keep its word and drop all pending court cases against protesters.
"I expect such action to be taken without delay so as to facilitate the political dialogue in (Ukraine's) parliament this week," she said.