The United States called for urgent U.N. Security Council talks on Zimbabwe because it said President Robert Mugabe had ignored international calls to end political violence ahead of a presidential election run-off, Reuters reported.
Zimbabwean police arrested opposition leader Morgan Tsvangirai twice on Thursday, making a total of four times in about a week. Tsvangirai's spokesman said the arrests were part of a harassment campaign in the run-up to the June 27 election.
Tendai Biti, secretary general of Tsvangirai's Movement for Democratic Change (MDC), was also arrested and would face a treason charge that could carry the death penalty, police said.
White House spokeswoman Dana Perino, accompanying President George W. Bush on a visit to Rome, criticised the "continued use of state-sponsored violence in Zimbabwe and the regime's actions, including unwarranted arrests of opposition figures".
"We believe the time has come for the United Nations Security Council to take up immediately the issue to prevent further deterioration of the region's humanitarian and security situation," Perino told reporters on Thursday.
A group of prominent African leaders joined the international chorus for an end to political violence in Zimbabwe, once a regional bread basket but now in economic meltdown.
"It is crucial for the interests of both Zimbabwe and Africa that the upcoming elections are free and fair," former U.N. Secretary General Kofi Annan and 39 former African heads of state and civic leaders said in an open letter on Friday.
Diplomats said South Africa opposed U.N. Security Council involvement and was receiving support from China and Russia.
Zimbabwe has been hit by political unrest since a March 29 election in which Mugabe and his ZANU-PF party were defeated for the first time since independence from Britain in 1980.
The MDC, the main opposition party, accuses the government of organising attacks on its supporters and says 66 of its followers have been killed since March.
Mugabe and ZANU-PF blame the bloodshed on the opposition.
The MDC and other opposition activists say they fear ZANU-PF will try to rig the results of the run-off in a bid to extend Mugabe's 28-year rule. The Zimbabwean leader and his officials are accused of stealing past elections.
The Southern African Development Community, a grouping of 14 nations including Zimbabwe, has sent a team of election monitors to Harare. Observers from Western nations critical of Mugabe's government are not being allowed into the country.
The political turmoil has compounded Zimbabwe's economic woes. Inflation has soared to more than 165,000 percent, unemployment is around 80 percent and food and fuel shortages are commonplace.
Millions have fled to neighbouring countries in search of food and work.
Mugabe's critics blame Zimbabwe's decline on his policies, including the seizure of thousands of white-owned farms which they say has contributed to the collapse of agriculture since 2000.
Some of the most fertile farms have gone to leading supporters of Mugabe.