Yanukovych, who has not been seen since he left Kiev, was reported to have described the latest developments as a "coup", according to Interfax news agency. He said he would not resign or leave the country, and that all decisions taken by Parliament were "illegal".
Activists were unopposed by police as they roamed downtown Kiev, where clashes this week killed at least 77 people, venturing into the presidential compound in the city’s outskirts.
In parliament, lawmakers appointed a new speaker to coordinate the government’s activities until a new cabinet is named and replaced the head of the interior ministry, which is responsible for security forces.
Yanukovych, who travelled to the eastern city of Kharkiv after signing a deal with the opposition to end the bloodiest episode of the country's post-World War II history, has made no official comment. Protesters were guarding key buildings in the centre of Kiev.
"It’s clear that the president has lost all power and influence," said Yuriy Yakymenko, an analyst at the Razumkov Center for Economic and Political Studies in Kiev.
"The country is being ruled without him and parliament is now the center of decision making. The main task now is to form a new government, because the economic situation is very difficult, time is counted in hours."
The peace pact, announced on Friday after all-night talks in Kiev with European Union foreign ministers, envisions a national unity government within 10 days.
Lawmakers approved a return to the 2004 constitution, which would curb Yanukovych’s authority, and voted to free jailed former prime minister Yulia Tymoshenko. Under the constitution, the president still needs to sign the measures.
The bloodshed prompted EU governments to threaten sanctions on Ukrainian officials and send envoys to hammer out the peace deal. The accord’s fate hinges on the official opposition’s ability to control the most radical anti-government activists.
The crisis erupted on Nov 21, when Yanukovych rejected an EU integration pact and opted instead for $15 billion of Russian aid. Violence intensified this week in Kiev amid frustration among protesters that their demands for snap elections and governance changes were being ignored.
Speaking by phone, Oleksiy Haran, a member of the demonstrators’ Maidan Council, emphasised the constitutional change and new government as "the key victory".
Even so, opposition leader Arseniy Yatsenyuk, who heads Tymoshenko's party, pointed to further tensions.
Protesters will stay at the Independence Square camp in Kiev that they built in November, according to Yatsenyuk, who said Yanukovych would be given no assurances on his fate.
Vitali Klitschko, the former world boxing champion who heads the UDAR party, said those responsible for the bloodshed would be held to account.
Pravyi Sektor, the radical nationalist group that has advocated violence and supplied activists to man the front lines of the protesters' defences, said any peace deal must include a removal of the current regime.
"Pravyi Sektor is not laying down its arms," the group’s leader, Dmytro Yarosh, said late last night from the stage in Independence Square, surrounded by ski-masked supporters and in front of the bodies of activists killed this week.
"The main demand, Yanukovych’s dismissal, was not met."
Yanukovych went to Kharkiv, a city about 480 kilometres east of Kiev, said a senior US State Department official after talking to Ukrainian Foreign Minister Leonid Kozhara.
Interfax news service reported the president may take part in a meeting of lawmakers from eastern and southern Ukraine, areas where he enjoys popular support.
In parliament, dozens of deputies from the president's ruling Party of Regions defected. More than 300 of the legislature's 450 lawmakers voted to revert to the 2004 constitution, fire acting Interior Minister Vitaliy Zakharchenko and free Tymoshenko from her seven-year sentence. There is no timeline for her release.
Presidential elections, which had been scheduled for March 2015, must be held by December, according to the agreement signed by Yanukovych and the opposition. The EU welcomed the deal as the only peaceful and democratic way out of the crisis.
"It is now the responsibility of all parties to be courageous and turn words into deeds for the sake of Ukraine’s future," EU President Herman Van Rompuy said in a statement.