The opposition Movement for Democratic Change accuses President Robert Mugabe of trying to sabotage Tsvangirai's campaign in order to preserve his 28-year hold on power.

Tsvangirai was released a few hours after being stopped by armed police and told to go to the police station at Esigodini, 40 km (25 miles) southeast of Zimbabwe's second largest city Bulawayo.

The party called Tsvangirai's detention "a shameless and desperate act". It said police had banned several planned campaign rallies because authorities could not guarantee the safety of party leaders.

"The regime must let the president do that which the people of Zimbabwe have mandated him and the MDC, to help restore the dignity of the people of Zimbabwe," it said in a statement.

Police spokesman Wayne Bvudzijena blamed the opposition for the incident on Friday.

"They refused to stop at a roadblock. They just crashed through the roadblock, led by their MP-elect in the area," he said.

Tsvangirai, who beat Mugabe in a March 29 election but failed to win the majority needed to avoid a second ballot, was detained on Wednesday and held and questioned by police for eight hours.

On Thursday, police stopped and held five U.S. and two British diplomats for several hours after they visited victims of political violence. Zimbabwe also barred relief agencies from doing work in the country, suffering economic ruin.

"It is almost as if the regime is sending out a message to the region, to the international community that it doesn't care, that it has no respect for life, it has no respect for the rule of law," MDC secretary general Tendai Biti told the World Economic Forum for Africa in Cape Town.

"The regime is increasing the decibels of insanity."

DIPLOMATIC ROW

Washington blamed the diplomats' detention firmly on Mugabe's government. The United States and former colonial power Britain accuse it of trying to intimidate Tsvangirai's supporters ahead of the election.

The opposition says 65 people have been killed in violence since the first round of voting. Mugabe says the opposition is responsible.

U.S. Ambassador James McGee, who was among those detained on Thursday, will lodge an official complaint in a meeting with Zimbabwe's foreign ministry, the U.S. embassy in Harare said on Friday. It was not clear when the meeting would take place.

Zimbabwean police said the diplomats had triggered the incident by failing to identify themselves when they were stopped at Chipadze outside the capital.

Mugabe's government suspended the work of all international aid agencies in the southern African nation on Thursday, saying some of them were campaigning for the opposition. Britain and the European Union condemned the suspension.

"I am deeply distressed to think that hundreds of thousands of people who depend on aid from the European Commission and others for their very survival now face an even more uncertain future," said EU aid commissioner Louis Michel, demanding the immediate lifting of the ban.

Zimbabwe, once one of Africa's most prosperous countries, has seen food production plummet since 2000 when Mugabe's government began seizing thousands of white-owned farms as part of a land redistribution programme to help poor blacks.

Many of the farms have ended up in the hands of Mugabe loyalists, and the country now faces chronic food shortages. It has had to rely on handouts and imports to feed its people.

Mugabe blames sanctions imposed by Western countries for the collapse of the once prosperous economy. The opposition says he ruined Zimbabwe through mismanagement.

The Southern African Development Community, a regional grouping of 14 nations, including Zimbabwe, is sending observers to monitor the run-off.