Mr Najib, 59, was sworn in as prime minister by Malaysia's king on Monday afternoon.
BBC: Malaysia's Anwar Ibrahim urges protest over poll defeat
At Wednesday's rally, Mr. Anwar's repeated attacks on Mr. Najib elicited wild applause from the crowd.
Supporting Prime Minister Najib's reforms should only improve their prospects at the next election.
Still, some observers said Mr. Najib had secured what looks like a comfortable victory.
Najib, who took office in 2009, embarked on a major campaign to restore his coalition's luster.
To that end, Najib has loosened the quota system that has limited foreign ownership.
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Najib also sees that direct foreign investment and ownership, as practiced by neighbor Singapore, spur growth.
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Mr. Najib knows this better than anyone, having benefitted from the ouster of Mr. Abdullah.
You wouldn't expect Anwar to make those kinds of concessions - or Abdullah or Najib.
Urbane and dapper, 45-year-old Najib was once aligned with Anwar, but past history will mean little.
"Najib is a shrewd observer of the political flow, " says a former UMNO MP.
The next election, says one of Mr Najib's former advisers, will thus be presidential in character.
It is a sign, he says, that the cabinet is not buying into Mr Najib's ideas.
Mr Najib Razak assumed office in the wake of the election, replacing Abdullah Ahmad Badawi.
BBC: Malaysian PM Najib Razak paves way for general election
Prime Minister Najib Razak pledged to make the economy based on merit rather than on race.
Malaysian Prime Minister Najib Razak, who had helped to broker the deal, was also present.
He was approached by Najib, then new to the pm job, to join the civil service.
The prime minister, Najib Razak, condemned the attacks, offered compensation, and stepped up security around churches.
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APCO's Kuala Lumpur representative through 2010, Paul Stadlen, now works in Prime Minister Najib's office.
Mr. Najib's falling popularity at home suggests his days as Prime Minister could be numbered.
Najib Tun Razak was moved from education to defense, where he had served before.
Though not accused himself, Mr Najib's prospects may be fatally damaged by the case.
Malaysian Prime Minister Najib Razak told reporters he could neither confirm nor deny the report.
But Education Minister Najib Tun Razak last week maintained this was necessary to prevent an over-reaction.
Divisions over Malaysia's vote is reigniting a years-old feud between opposition leader Anwar and Prime Minister Najib.
The allegations of vote fraud, though, if substantiated, could undermine Mr. Najib's credibility, especially among urban voters.
Prime Minister Najib recognizes that a resumption of economic growth is a large part of the answer.
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While Mr. Najib has adopted the outward trappings of a dynamic, youth-courting reformer, results have been mixed.
That set the stage for Mr. Najib to take over the party leadership and premiership in 2009.
Since Mr. Najib came to power in 2009, he has made efforts to make governance more transparent.
WSJ: Malaysia's Prime Minister Najib Razak Dissolves Parliament, Paving Way for Elections
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