The protesters appeared to have encountered no resistance and already begun erecting barricades outside the building with rubbish containers.
The Ukrainian opposition says a compromise deal offered by President Viktor Yanukovych is not enough to end the country's worst crisis since independence as nationwide protests spread to the president's eastern heartland.
With Ukraine shaken by a week of violence between police and protesters, all three main Ukrainian opposition leaders personally attended the Kiev funeral of one of three activists confirmed to have been shot dead in the unrest.
The protests began more than two months ago over Yanukovych's rejection of a pact with the European Union under Russian pressure but have now turned into an all-out bid to oust him from power.
Tensions remained high in Kiev as several dozen protesters seized control late Sunday of the Justice Ministry, smashed windows and erected barricades based around rubbish containers outside, an AFP correspondent reported.
Europe has urged dialogue between the two sides - a call echoed by Pope Francis who voiced hope in his weekly Angelus prayer on St Peter's Square that "the search for common good may prevail in the hearts of all".
Under unprecedented pressure, Yanukovych late Saturday offered the opposition posts in government including that of prime minister but his opponents said that the offer fell short of what they needed.
Yanukovych offered to share leadership with Fatherland party leader Arseniy Yatsenyuk as prime minister and UDAR (Punch) chief and world boxing champion Vitali Klitschko as deputy prime minister.
Klitschko, who is believed to have a personal rivalry with Yatsenyuk, branded the proposals "poisoned" in an interview with German newspaper Bild am Sonntag.
"This was a poisoned offer by Yanukovych designed to split our opposition movement," he was quoted as saying.
Opposition leaders have been careful, however, neither accepting nor explicitly rejecting Yanukovych's proposals. They have said talks will continue although it is not clear exactly when.
Yanukovych's office has also said the president is willing to consider constitutional changes to reduce his power and return to a system according more authority to the prime minister.
A crucial day in the standoff is expected to be Tuesday when parliament will meet in extraordinary session to debate key sticking points in the crisis, including possible changes to key protest laws.
Yanukovych has notably failed to respond to the key opposition demand of bringing forwards presidential elections due in 2015 and has also shown no sign of releasing jailed opposition leader Yulia Tymoshenko.
"We want the authorities to understand that we will stay until victory, and most of us see that as the departure of Yanukovych," said a protester called Bogdan.
Thousands of activists meanwhile laid siege to local government offices in four Ukrainian cities including the eastern hub of Dniepropetrovsk, Sumy in the northeast and Zaporizhya in the southeast that have in the past been sympathetic to Yanukovych.
Police used batons and stun grenades to break up the rally in Zaporizhya, causing injuries, local media said.
Protesters have already occupied regional administrations in 10 Ukrainian regions to protest against Yanukovych-appointed governors.
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