U.N. High Commissioner for Human Rights Louise Arbour said she was shocked and concerned about the brutal attacks against political activists that opposition leader Morgan Tsvangirai said have led to more than 50 deaths.

Official figures showed Tsvangirai beat President Robert Mugabe in the March election, but did not garner enough votes to avoid a second round poll, set for June 27.

Tsvangirai's Movement for Democratic Change (MDC) accuses Mugabe's ZANU-PF party of launching a violent campaign against its supporters since the election to avoid defeat next month. The ruling party denies that charge and blames the MDC for the violence.

In a statement released in Geneva, Arbour referred to reports that the bodies of a number of slain MDC activists, including two who were abducted by armed men, had been discovered over the past week in Zimbabwe.

"It is hard to get a very precise picture of the full range of the violence, or the exact number of politically motivated extra-judicial killings," Arbour said.

"There appears to be an increasing pattern of people being targeted for politically motivated assassination," she said.

"Arrests, harassment, intimidation and violence -- directed not just at people with political affiliations, but also at members of civil society -- are continuing on a daily basis."

The former Canadian Supreme Court justice urged authorities in Zimbabwe to investigate and prosecute abusers, and "to take urgent steps to protect all the country's inhabitants from further attacks in order to create an atmosphere conducive for a free and fair presidential election" in June.