Veteran government lawmaker Razaleigh Hamzah, who wants to challenge Prime Minister Abdullah Ahmad Badawi for the leadership of the long-ruling United Malays National Organization, said the results meant that "what scraps of credibility (Abdullah) had left" are gone.
Abdullah, however, played down the significance of Anwar's triumph.
"I believe we can still continue the government," the prime minister was quoted as saying by the Bernama national news agency.
"What happened ... was not something so big as to change the situation that exists after the last general election."
Abdullah's party has been the main party in a coalition that has governed Malaysia uninterrupted since independence from Britain in 1957, but which was seriously weakened for the first time in the March elections.
Anwar has said he aims to bring the government down by mid-September via defections to his opposition group by unhappy lawmakers in coalition parties.
In the March elections, Anwar's three-party alliance won an unprecedented 82 of parliament's 222 seats - 30 short of a majority - and wrested control of five states.
Parliament Speaker Pandikar Amin Mulia said Anwar would be formally sworn in as a lawmaker Thursday. Opposition parties also planned to endorse him as their leader ahead of a parliamentary session Friday, when Abdullah announces the annual budget.
Former Prime Minister Mahathir Mohamad, who has slammed his successor's policies, said he believed many government supporters voted for Anwar "so that Abdullah will realize that his leadership is no longer wanted."
"Abdullah must take responsibility and resign now," Mahathir said.
Mukhriz Mahathir, Mahathir's lawmaker son, said that "with Anwar Ibrahim in Parliament, we cannot afford to have a weak leadership because it could lead to our downfall."
"The walls are crumbling but the top guy seems oblivious to his surroundings," he said.
Government supporters also vented their fury on Internet forums.
Mykmu Net, a Web site for ruling party members, published comments by readers who said they "hope (Abdullah) will be quickly ousted" and that Abdullah's "resignation will be the only way out."
Abdullah resisted calls to resign after the March elections, though he has pledged to hand power to his deputy, Najib Razak, by mid-2010 in a protracted transition plan publicly endorsed by most top government officials.
Anwar's re-entry into Parliament will complete his political rehabilitation.
He was fired by Mahathir as deputy prime minister in 1998 and jailed for six years after he was convicted of corruption and sodomizing his family driver. The sodomy conviction, which Anwar charged was a politically motivated frame-up, was overturned by Malaysia's top court in 2004.
In June, a new sodomy charge was leveled against Anwar, this time involving a 23-year-old man, and government leaders mounted a fierce by-election campaign against Anwar hoping the case would prompt voters in the charismatic politician's longtime stronghold to desert him.
Anwar, who pleaded innocent, said the new "most sickening" allegation was politically motivated. No date has been set for a trial.
