By Nelson Banya

HARARE, May 18 (Reuters) - Zimbabwe's main opposition party remains committed to negotiations with the government despite an intensified crackdown in which many of its members have been arrested or detained, a party official said on Friday.

The Movement for Democratic Change (MDC) says more than 600 opposition supporters have been abducted and tortured by government agents since February. It says 150 activists and leaders, including party president Morgan Tsvangirai, have sustained serious injuries.

President Robert Mugabe's government accuses opposition activists of unleashing violence in the townships and engaging in "terrorist" activities.

MDC secretary general Tendai Biti said the government was trying to weaken the opposition ahead of general elections next year through the arrest and detention of its key officials, but reported progress in the negotiations being mediated by South African President Thabo Mbeki.

"As a party, we fully welcome the initiative and we believe in the process, but our first demand is, you cannot have dialogue and elections in a situation of violence and the attack on the (opposition) party must stop if we're to have any meaningful discourse," Biti said.

Mbeki told the South African parliament on Thursday that negotiations between the MDC and Mugabe's ruling ZANU-PF "were progressing well."

Biti said the MDC had launched a "Free Them Now" campaign to press the government to release detained opposition activists.

"We have at least 32 key members currently incarcerated at Harare Remand Prison, facing totally fabricated and trumped-up charges designed to paralyse the party," Biti said. Among those arrested is the MDC's elections chief, he said.

"With the elections technically six months away, how do you expect the party to organize its campaign when its elections director is detained? We are institutionally crippled," Biti said.

The MDC planned to take the Free Them Now campaign to the African Commission for Human and People's Rights, now sitting in Ghana, and to the Southern African Development Community's Parliamentary Forum.

He vowed the party would defy Mugabe's government, which has banned all rallies and protests in central Harare as well as opposition strongholds in the townships.

"If they think they will cow us, they will not. The choice for Mugabe is simple: either you give us freedom or we fill all the prisons with our activists. We are prepared to pay the ultimate price until we get our freedom," Biti said.

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