Ukraine's opposition has asserted its authority over Kiev and parliament in a day of fast-paced events.
MPs have replace the parliamentary speaker and attorney general, appointed a new pro-opposition interior minister and voted to free jailed opposition leader Yulia Tymoshenko.
Police appear to have abandoned their posts across the capital.
Protesters in Kiev have walked unchallenged into the president's official and residential buildings.
President Viktor Yanukovych and opposition leaders signed a peace deal on Friday after several days of violence in which dozens of people died in a police crackdown on months of protest.
But the deal failed to end the protests and huge crowds remain in Independence Square, the Maidan.
The opposition have called for elections before 25 May, earlier than envisaged in Friday's peace deal.
The president's whereabouts are unclear - his aides say he is in Kharkhiv, close to the border with Russia.
Presidential aide Hanna Herman said he was due to give a televised address later.
A gathering of deputies from the south-east and Crimea - traditionally Russian-leaning areas - is taking place there, but Ms Herman said the president had "no intention" of attending, nor of leaving the country.
An opposition figure has announced to the protest crowds in Independence Square that the president has resigned. This has not been confirmed, but the crowds reacted with huge cheers.
Ms Tymoshenko was sentenced to seven years in prison in 2011 for abuse of power. Her supporters had always maintained this was simply Mr Yanukovych taking out his most prominent opponent, and her release has always been a key demand of the protest movement.
She is expected to be released in Kharkiv later on Saturday.
Her daughter, Yevheniya Tymoshenko, said she was thankful "to all Ukrainians, the democratic world, and the lawmakers who have just released her".
"We must go there now very quickly to ensure that nothing happens to her and to free her together," she said.
On Saturday morning as parliament met, speaker Volodymyr Rybak resigned, citing ill health. He has been replaced by Oleksandr Turchynov, an ally of Ms Tymoshenko.
Another Tymoshenko ally, Arsen Avakov, has been appointed interim interior minister. He replaces Vitaly Zakharchenko, who was sacked on Friday after being blamed for the deaths of civilians in last week's crackdown on protests.
Vitaly Klitschko, leader of the opposition Udar party, repeated his demand for the president's immediate resignation.
The protests first erupted in late November when Mr Yanukovych rejected a landmark association and trade deal with the European Union in favour of closer ties with Russia.
On Thursday, in the worst violence since the unrest began, police opened fire on protesters who were occupying Independence Square in central Kiev. The health ministry says 77 people - both protesters and police - have been killed since Tuesday.
For a second day, funerals are being held in the square.
The BBC's Kevin Bishop in Kiev says journalists and protesters were able to enter freely the previously heavily guarded presidential complex.
The protesters have not entered the offices themselves. They said they were protecting the buildings from looting and vandalism.
"He's not here, none of his officials or anyone linked directly to the administration are here," said Ostap Kryvdyk, a protest leader, referring to the president.
Correspondents say police appear to have abandoned posts across the city, while the numbers gathered in the Maidan are growing.
Hundreds of people have also entered the grounds of the president's official residence, the Mezhyhirya, about 15km (10 miles) north of the city centre.
In a statement, the interior ministry said the police force was "at the service of the people and completely shares its aspirations for rapid changes".
"We pay homage to the dead," it added.
The political pact was signed on Friday by President Yanukovych and opposition leaders after mediation by EU foreign ministers, and approved by Ukraine's parliament.
It restores the 2004 constitution - reducing the powers of the presidency - and says a unity government will be formed and elections held by the end of the year.
All but one of the 387 MPs present voted in favour, including dozens of MPs from Mr Yanukovych's own Party of Regions.
The deal has been met with scepticism by some of the thousands of protesters who remain in the square. Opposition leaders who signed it were booed and called traitors.
The US and Russian presidents have agreed that the deal needs to be swiftly implemented, officials say.
Russia's Vladimir Putin told Barack Obama in a telephone conversation on Friday that Russia wanted to be part of the implementation process, a US state department spokesperson said.