A leading human rights group accused Zimbabwe's army Wednesday of working with ruling party militants to unleash "terror and violence" against dissent. New York-based Human Rights Watch joined other rights groups and Zimbabwe's opposition party in linking violence since last month's presidential vote to the security forces and so-called "war veterans" - groups loyal to autocratic President Robert Mugabe. Mugabe's regime has countered that the opposition groups are responsible for the violence, even arresting scores of people last week, including women and their nursing babies, who the opposition says had taken shelter from the violence at its headquarters in Harare, Zimbabwe's capital.

Lawyers for Zimbabwe's opposition Movement for Democratic Change said the Harare High Court ordered late Monday that everyone arrested at its headquarters last week should be freed. On Tuesday, police released the last of those still being held.

Mugabe has been accused of using violence and intimidation and plotting fraud to hold onto power after March 29 presidential elections he is believed to have lost.

Results from those elections have yet to be released, and ruling party officials have said a runoff would likely be necessary. Opposition candidate Morgan Tsvangirai says he won outright, though independent observers say Tsvangirai won the most votes, but not the 50 percent plus one vote needed to avoid a runoff.

"The army and its allies - 'war-veterans' and supporters of the ruling party Zanu-PF - are intensifying their brutal grip on wide swathes of rural Zimbabwe to ensure that a possible second round of presidential elections goes their way," Georgette Gagnon, Africa director at Human Rights Watch, said in the group's statement released Wednesday.

Neither the army chief nor a government spokesman could be reached for immediate comment.

"The African Union and U.N. Security Council should take immediate steps to help prevent a further escalation in violence," Gagnon said.

The opposition also has appealed to African leaders and to the U.N. for help.

On Tuesday, a Zimbabwe opposition leader urged the U.N. Security Council to appoint a special envoy to his country. But the deeply divided council, which was briefed on situation behind closed doors, took no action.

While the United States, Britain and France back council engagement and sending a U.N. envoy to Zimbabwe, diplomats said South Africa, Russia, China and other members oppose any action now.

In the hearing, Britain's deputy U.N. ambassador Karen Pierce said U.N. political chief B. Lynn Pascoe had spoken of "a level of political intimidation and violence that I think many council members found quite chilling."

U.S. deputy ambassador Alejandro Wolff said he was struck by Pascoe's characterization of the situation in Zimbabwe "as the worst humanitarian crisis since independence" in 1980.

Human Rights Watch said war veterans were being given guns and trucks by the army and were "collaborating with the army in unleashing a campaign of terror and violence against (opposition) MDC members and supporters."

The veterans groups are fiercely loyal to Mugabe and have roots in the nation's independence struggle but increasingly include militants too young to have been guerrilla fighters.

In one example in the eastern province of Manicaland, Human Rights Watch said, opposition members went to an "informal torture center" set up by war veterans and saw 12 of their colleagues who had been abducted, bound and beaten. The war veterans fired on those who had come asking questions, hitting three and killing one as they fled. The current whereabouts of the 12 abductees were not known, Human Rights Watch said.

"When the victims informed the local police about the incident, the police refused to take action, claiming that such an incident could not have happened since they had no knowledge of any civilians in the area who were allowed to keep firearms," Human Rights Watch said.