Zimbabwean police have detained opposition leader Morgan Tsvangirai for the second time this week.
His party, the Movement for Democratic Change, said police blocked Mr Tsvangirai from reaching a campaign rally for the presidential run-off vote.
Police were not immediately available for comment.
Mr Tsvangirai was earlier stopped from reaching a rally outside Zimbabwe's second largest city, Bulawayo, after police put up a roadblock. The MDC accuses President Robert Mugabe of trying to sabotage his campaign.
The MDC said earlier that harassment of diplomats and aid groups showed Mr Mugabe's government would fail to respect the rule of law during the 27 June vote.
The accusation came a day after police detained US and British diplomats outside Harare and relief agencies were barred from doing work in the country.
'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,' he said.
Mr Tsvangirai beat Mugabe in a 29 March election but failed to win the majority needed to avoid a second ballot, according to official results.
Earlier this week he was held and questioned by police for eight hours.
Yesterday police stopped and held five US and two British diplomats for several hours after they visited victims of political violence, prompting strong condemnation from the US and British governments.
Washington blamed the diplomats' detention firmly on Mr Mugabe's government, which the US accuses of trying to intimidate Mr Tsvangirai's supporters ahead of the election.
US ambassador James McGee, who was among those detained, will lodge an official complaint in a meeting with Zimbabwe's foreign ministry, the US embassy in Harare said. 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.
Mr Mugabe's government suspended the work of all international aid agencies in the southern African nation yesterday. Britain and the EU condemned the suspension.
'We utterly condemn the behaviour of the Zimbabwe regime and the damage they are causing to the welfare of their own people,' said a spokesman for British Prime Minister Gordon Brown.
'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, which was once one of Africa's most prosperous countries, has seen food production plummet since 2000, when Mr 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.
Zimbabwe has accused CARE International and other non-governmental groups of political involvement, including campaigning for the MDC. CARE and others deny the charges.
The US and British governments, along with human rights groups and Zimbabwe's opposition, have accused Mr Mugabe of a campaign of violence to try to keep his 28-year hold on power. Mr Tsvangirai says 65 people have been killed.
The opposition accused the government of waging genocide.
'What is happening in Zimbabwe is moving towards the direction of full-scale genocide... Robert Mugabe and the state of Zimbabwe are committing state-sponsored violence, murder and torture to stay in power,' Arthur Mutumbura, head of a smaller MDC faction, told the Cape Town forum.
Mr Mugabe blames his opponents for the violence and 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.
