Gordon Brown has warned Zimbabwe leader Robert Mugabe that he is "appalled" at the latest developments in the country.
Zimbabwean police have banned political rallies amid growing tension over the disputed presidential election.
The prime minister said the world was running out of patience with President Mugabe, with results still not released almost two weeks after the election.
Mr Brown was speaking ahead of a regional summit in Zambia to discuss the crisis.
He 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."
Mr Brown added: "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".
Members of the 14-nation Southern African Development Community (SADC) met in the Zambian capital Lusaka on Saturday.
Despite growing tensions, Mr Mbeki said there was "no crisis" in Zimbabwe.
Last weekend Mr Brown held private talks with South African President Thabo Mbeki, spending more than two hours trying to persuade him to use his influence to end the crisis.
In Zimbabwe, the opposition Movement for Democratic Change (MDC) - which claims to have won the election - has called for a strike starting on Tuesday to put pressure on the authorities.
The ban on political rallies comes two days before a planned demonstration by the MDC in the capital Harare.