The announcement raised the prospect that the persistent confrontation between Saakashvili and his opponents will be played out in the streets instead of in the legislature, denting the president's effort to portray Georgia as a stable democracy in Western eyes.
"We refuse to enter parliament -- to enter that building," Levan Gachecheladze, co-leader of the United Opposition, told a crowd outside the parliament building in Tbilisi.
He called on supporters to gather Monday, Georgia's Independence Day, for a street protest.
A nearly complete vote count indicated that the United Opposition won just 16 of the 150 parliament seats in Wednesday's election, the Central Election Commission said. Commission spokesman Zurab Kachkachishvili said Saakashvili's United National Movement would hold about 120 seats.
That would give the governing party an even stronger grip than it had over the previous 235-seat parliament after it suffered defections as disgruntled Saakashvili allies went into the opposition.
Saakashvili's opponents disputed the results, claiming widespread violations.
"We do not accept the legitimacy of these elections due to total falsification, so it's not ruled out that we will refuse to enter parliament," David Gamkrelidze, the other top United Opposition leader, told reporters.
The growing discord between Saakashvili and his opponents has spilled into the streets repeatedly over the past year, heightening tension in the nation at the center of a struggle for influence between Russia and the West.
An election-night rally drew just a few thousand people, adding to questions about the opposition's ability to hold the kind of large, sustained protests that could force political change.
It was not immediately clear whether the other three opposition parties would follow the United Opposition and refuse to take the 14 seats they were on track to win. The leader of the Labor Party said he would do so only if all opposition forces joined the boycott.
The opposition cannot count on the West for support of an effort to contest the overall vote, despite some criticism from international observers of an election portrayed as a test of Saakashvili's commitment to democracy that could affect his NATO membership drive.
In a statement issued Friday by its presidency, the European Union noted that the main international observer mission said implementation of Georgia's democratic commitments was "uneven and incomplete" and urged authorities to ensure that all complaints are "urgently addressed."
But the EU also urged "all political forces to respect the election results."
Meeting in Brussels with EU foreign policy chief Javier Solana, Georgian Foreign Minister Eka Tkeshelashvili said the government did all it could to ensure a free and fair election and promised to address the problems raised by observers.
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