Prime Minister Najib Razak's Barisan Nasional (National Front) coalition won May 5 elections but the opposition, which gained more than half the popular vote, has said ruling-coalition fraud cost them victory.
Anwar has rejected the outcome and led a series of rallies denouncing it as fraudulent, raising the spectre of prolonged instability over the bitter elections, the closest the 56-year-old ruling coalition ever came to losing.
But he said his three-party opposition would participate in parliament "as a reality for now" while continuing to question the government's legitimacy.
"They stole the election from us," he said.
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The opposition says voter rolls were full of irregularities, enabling the ruling coalition to sway results in closely fought seats. Supposedly indelible ink introduced to prevent multiple voting easily washed off.
The government has admitted "problems" with the ink -- which it had touted as proof of its commitment to fair polls -- but has rejected charges of cheating.
The election was seen as the first in which Barisan Nasional, which has governed tightly since independence in 1957, faced possible defeat.
The opposition plans to file petitions in some 30 constituencies challenging the results. If won, they say it would give them control of parliament.
But such petitions rarely succeed and the opposition has failed to produce evidence of widespread fraud.
Anwar said the opposition would focus on pushing for reform of an electoral system widely seen as unfairly favouring Barisan.
Over the weekend, Najib said a bipartisan special parliamentary panel would be set up to oversee the Election Commission to fight perceptions it is biased.