The international community's patience with Zimbabwe's president, Robert Mugabe, is wearing thin, Gordon Brown warned last night, adding that he was "appalled" at the latest developments in the country.

Earlier, Zimbabwe's state radio station announced a ban on political rallies as the result of the presidential election continued to be withheld, almost two weeks after the poll Mugabe is believed to have lost.

On the eve of a summit of African leaders in Zambia to discuss the crisis, Brown said: "The Zimbabwean people have demonstrated their commitment to democracy. We, and the leaders of the region, strongly share this commitment.

"I cannot understand why it is taking so long to announce the result of the presidential elections. I am appalled by the signs that the regime is once again resorting to intimidation and violence.

"We will be vigilant. The international community will remain careful to do nothing to undermine efforts to secure an outcome that reflects the democratic will of the people of Zimbabwe. But the international community's patience with the regime is wearing thin."

Mugabe is expected to snub the summit in Zambia.

As tension rose over the withheld results, Zimbabwe police accused the opposition Movement for Democratic Change of "spoiling for a fight" and of deploying 350 youth wing members around the country. The police banned a rally planned for tomorrow by the MDC, which has called a general strike for Tuesday to push for results to be released.

An opposition spokesman, Nelson Chamisa, said: "You can't be a political organisation and not hold a rally, that is what makes us who we are, how we communicate with our supporters."

He added that the party had not decided whether to go ahead with the rally. "We cannot accept a declaration of a police state. People have just voted for change, for democracy and what do they get? This is unacceptable. This is ridiculous."

State radio said Zimbabwe would be represented by three ministers at the summit on Saturday of the Southern African Development Community (SADC), which was expected to press Mugabe to issue results from the March 29 election.

State radio said demands for Mugabe to release the results were misplaced because that was the prerogative of the Zimbabwe electoral commission.

Human rights organisations and the MDC say Mugabe has unleashed a campaign of systematic violence in response to his Zanu-PF party's first electoral defeat, when it lost control of parliament in the March 29 election.

The MDC says its leader, Morgan Tsvangirai, won the presidential vote, and that Mugabe's 28-year rule is over.

It accuses Mugabe of delaying the result so that he can intimidate opposition supporters before a runoff vote against Tsvangirai. Zimbabwean police said all political rallies had been banned because officers were too busy guarding ballot boxes or deployed to prevent post-election violence.

Amnesty International said there were widespread incidents of violence in Zimbabwe "suggesting the existence of coordinated retribution against known and suspected opposition supporters".

Amnesty called on SADC leaders to redouble efforts to avoid further deterioration of the human rights situation and urge the electoral commission to release the results.

Human Rights Watch said the Lusaka meeting was SADC's "last real chance" to resolve the crisis in Zimbabwe. The US-based body also accused Zanu-PF of increasing assaults on opposition activists and polling agents.