Mr Anwar said the vote was a critical point in Malaysia's history
Malaysian opposition leader Anwar Ibrahim has won a landslide victory in a by-election in Penang.
A high turnout saw his People's Justice Party winning 31,195 of 47,000 votes, the Election Commission said.
The result returns Mr Anwar to parliament after a decade's absence, and comes despite sodomy charges he says are politically motivated.
The 61-year-old has been trying to make his political comeback since a ban on seeking public office expired in April.
He had been expected to win but the level of support for him was substantial - gaining almost double the votes of his opponent, reports the BBC's Robin Brant from Penang.
Mr Anwar told the BBC the overwhelming by-election victory was a "key point of departure" for his coalition in its campaign to bring down the government.
"It is an endorsement of our policies and a rejection of the obsolete policies, the corrupt policies of the government," he said.
He added that Malaysia's embattled Prime Minister Abdullah Badawi "must take heed of the sentiments and the call of Malaysians".
Mr Anwar defeated Arif Shah Omar of the ruling Barisan Nasional (National Front) coalition.
Although only a single seat was at stake, the event transfixed Malaysians. While the result will not change the balance of power, it will strengthen Mr Anwar's hand in his campaign to become the next prime minister, our correspondent adds.
Thousands of rival supporters had flooded into the small area of Permatang Pauh to try to help or hinder Mr Anwar, and there was a heavy police presence in Penang as votes were cast.
"The Malaysians consider this a critical point in our history, whether we get the mandate to chart a new course for Malaysia or we should just be happy with the same corrupt establishment," Mr Anwar told reporters before the vote.
Deputy Prime Minister Najib Razak, who lead the ruling party's campaign, told the BBC the vote represented a "titanic struggle".
One senior government politician even described it as a "jihad".
Observers said government-controlled newspapers had led an assault on Mr Anwar, emphasising the sodomy allegations against him.
Voting irregularities were uncovered, and several Israeli flags were also put up - in an apparent attempt by government supporters to persuade the mainly Muslim voters that the opposition leader was sympathetic to the Jewish state.
Mr Anwar was sacked as deputy prime minister in 1998, and jailed for six years for sodomy and corruption - allegations he always denied.