He made the claim just before police broke up the rally held to celebrate the end on Monday of a five-year ban that had kept him from running for political office.
"Now I can say for the first time that we are ready to govern the country," Anwar said.
He said parliamentarians belonging to the ruling coalition were willing to defect to the opposition.
"We have the numbers ... some [government legislators] have had discussions with us, but we are not in a hurry."
Organisers estimated 40,000 people gathered to hear Anwar's first public speech in the capital, Kuala Lumpur, since last month's general elections.
Police said the number was closer to 4,000.
"We are just waiting for the right time. We want to create a new era for Malaysia," Anwar said to applause from the crowd.
"We will hand over the parliamentary opposition post to Prime Minister Abdullah Ahmad Badawi."
Police halted Anwar an hour into his speech and the crowd dispersed peacefully.
Police said the rally was illegal because it did not have a permit required by Malaysian law for all public assemblies of more than five people, and had urged people to stay away.
Officially Malaysia's opposition coalition has 82 seats in the 222-member parliament but unofficially has enough parliamentary members to form a government, Anwar said before police surrounded the rostrum where he was speaking.
Anwar was cast into the political wilderness after being dropped in 1998 by Mahathir Mohamad, Malaysia's prime minister then and, originally, a mentor of Anwar's.
But his political fortunes have been revived by the opposition's gains in last month's general elections.
For the first time in 40 years, the opposition prevented the ruling Barisan Nasional coalition from taking a two-thirds parliamentary majority and got control of five states.
Barisan's poor showing sparked some calls for Abdullah to resign, although he has refused to step down.