Malaysian opposition leader Anwar Ibrahim was acquitted Monday in a stunning conclusion to a two-year sodomy trial that he had condemned as a conspiracy to cripple his resurgent political alliance.

"Thank God, justice has been served," Anwar told reporters after being cleared of sexual misconduct with a young male aide, and pledged to topple the government in national polls expected to be held this year.

"In the coming election, voice of the people will be heard and this corrupt government will be toppled from its pedestals of power," he added in a Twitter posting.

The unexpected decision set off pandemonium at the Kuala Lumpur High Court where Anwar was mobbed by his wife, daughters and opposition politicians.

Thousands of opposition supporters who had gathered outside under heavy security erupted into cheers and raised their fists in the air.

In his brief verdict, Judge Mohamad Zabidin Diah said he could not rely on controversial DNA evidence submitted by the prosecution in the case lodged by Anwar's aide Mohamad Saiful Bukhari Azlan.

"The court is always reluctant to convict on sexual offences without corroborative evidence. Therefore, the accused is acquitted and discharged," he said.

The verdict in the long-running trial defied the expectations of many political observers and even Anwar himself, who had said the government of Prime Minister Najib Razak was intent on eliminating him as a political threat.

Information minister Rais Yatim said: "Malaysia has an independent judiciary and this verdict proves that the government does not hold sway over judges' decisions.

"The current wave of bold democratic reforms introduced by Prime Minister Najib Razak will help extend this transparency to all areas of Malaysian life," he said in a statement.

Najib faces a deadline of early next year to hold new polls in the ethnically diverse and resource-rich nation, but he is expected to call them sooner.

With his name cleared, Anwar can now campaign freely for the polls at the helm of his opposition alliance -- which brings together his multi-racial Keadilan, a conservative Islamic party, and a liberal Chinese-based party.

Ahead of the verdict, hundreds of police officers and other security guarded the streets around the high court, which was cordoned off, and a water cannon truck was parked nearby.

Police had given rare consent for the courthouse rally. Thousands had taken to the streets after Anwar was first ousted from office in the 1990s, in unprecedented demonstrations.

It was the second sodomy verdict in a dozen years for Anwar, a former deputy premier in the 1990s who was next in line to head the country's Barisan Nasional coalition government until a spectacular downfall.

The charismatic Anwar had been groomed to succeed then prime minister Mahathir Mohamad until a bitter row between them forced Anwar out in 1998, and jailed on sodomy and graft charges widely seen as politically motivated.

Once the sodomy charge was overturned in 2004 and he was released from prison, Anwar reinvented himself as head of the opposition which he led to unprecedented gains in 2008 general elections.

The new sodomy charges emerged shortly after those polls, sparking accusations they were concocted by the government to stall the opposition revival which threatened its half-century grip on power.

Sodomy is illegal in Muslim-majority Malaysia and punishable by up to 20 years in jail. Phil Robertson, deputy Asia director of Human Rights Watch, said after Monday's verdict that the charge should never have been brought.

"Hopefully this verdict sends a message to the government to put this matter to rest," he said in a statement.