The reform package, approved in a 402-21 vote with 19 abstentions, paves the way for a presidential re-vote on Dec. 26 between Prime Minister Viktor Yanukovich, who was declared the official winner, and his opposition rival Viktor Yushchenko, who alleged massive voter fraud in the November election.
After reviewing the opposition's case, the Supreme Court agreed with Yuschenko's supporters, annulling the election and ordering the new ballot in December.
Lawmakers stood and cheered as President Leonid Kuchma signed the reform package.
Yushchenko told reporters outside parliament, "Today's decision opens the way for my victory in 18-20 Ukrainian regions. The main thing is today we managed to achieve what we had hoped for. This is a day of historic compromise."
The United States and other Western governments also praised the decision, which followed weeks of protests by Yushchenko supporters in Kiev during which they blocked many government buildings.
"Ukrainians are coming together to find a Ukrainian solution," Secretary of State Colin Powell said in a speech in Brussels, where he was attending NATO talks, the Associated Press reported.
Demonstrators will end their blockade of government buildings by Wednesday night, said Socialist Party leader Oleksandr Moroz, a key Yushchenko ally.
But Yanukovich, campaigning in eastern Ukraine, said he was "not happy" with the vote, describing it as a "soft coup d'etat."
"If we win the new vote and they try to deal with us in the same way, we will simply have to defend our victory," he said, according to Reuters.
To prevent electoral fraud in the new ballot, the newly approved reforms place restrictions on absentee voting.
Kuchma also said he would accept the resignation of the prosecutor general, a key opposition demand.
The reform package also increases parliamentary powers over the president. The president no longer has the power to appoint his own government, but keeps the right to reject parliamentary nominees for the top three posts -- prime minister, foreign minister and defense minister, the AP reported.
And parliament now has the right to appoint all other Cabinet positions without presidential approval.
-- Compiled from wire reports and other media sources