HARARE (Sapa-AFP) - South African President Thabo Mbeki on Friday held intensive talks with veteran counterpart Robert Mugabe over Zimbabwe's post-election crisis as doctors reported a dramatic rise in violence.

Mbeki, the region's chief mediator on Zimbabwe, held two rounds of discussions with Mugabe lasting more than three hours after arriving in Harare for his first visit since the announcement of presidential election results.

He did not meet any other Zimbabwe officials or opposition representatives and left the country late in the evening without commenting to reporters.

"President Mbeki was here to get an understanding of the situation and developments in the country after the harmonised elections (in March) and the current preparations (for a run-off election)," said Zimbabwe's information minister, Sikhanyiso Ndlovu, as Mbeki headed home.

Mbeki held three hours of talks with Mugabe after arriving in Harare, before returning later in the day for another half-hour after consultations at the South African embassy, according to an AFP correspondent outside the venue.

The opposition Movement for Democratic Change (MDC), which won control of parliament and whose leader won a first-round victory against Mugabe in polls on March 29, has called for Mbeki to be axed as a mediator over his softly-softly approach towards the Zimbabwean leader.

As the two leaders held talks, a coalition of doctors said there had been a dramatic escalation in attacks in rural areas by supporters of Mugabe.

The main labour federation, meanwhile, said its two top leaders had been arrested over speeches made to workers at a May Day rally.

In the latest of its regular updates, the Zimbabwe Association of Doctors for Human Rights said it had received reports of more than 900 cases of organised violence and torture, but said the real figure was
much higher.

The association said the violence carried echoes of the violent build-up and aftermath of the 2002 election when Mugabe was last re-elected.

"However, the current violence is dramatically more intensive and unrestrained," it said. "The level of brutality and callousness exhibited by the perpetrators is unprecedented."

The figures were released as the US ambassador to Harare, James McGee, led a group of diplomats on a tour of victims of political violence at a private clinic in Harare.

Ages of the victims ranged from four years old to over 80, according to an embassy statement.

The MDC has said 30 of its supporters have been killed since election day and thousands more have been tortured or injured.

Those figures have been strongly disputed by the Zimbabwean authorities who have accused the opposition of being behind violence, such as arson attacks.

The authorities have been rounding up an increasing number of high-profile opponents, including the veteran editor of one of the country's few remaining independent newspapers on Thursday.

A spokesman for the Zimbabwe Congress of Trade Unions said its president Lovemore Matombo and secretary-general Wellington Chibebe had also been arrested allegedly "for inciting people to rise up against the government" during speeches at a May Day rally.

The organisation was formerly headed by Morgan Tsvangirai, now MDC leader and Mugabe's rival in any presidential election run-off.

Tsvangirai, who insists he won an absolute majority on March 29, was due to announce at a press conference in Pretoria on Saturday whether he would contest another round of voting.

The electoral commission, which took nearly five weeks to announce the results of the first round, has yet to set a date for the run-off.

Tsvangirai has been out of Zimbabwe for several weeks, meeting with African leaders and diplomats in an effort to step up pressure on the 84-year-old Mugabe to step down after 28 years in power.

Mbeki has consistently refused to publicly criticise Mugabe, even though South Africa has paid a high price for the economic meltdown across its northern border.

Some three million Zimbabweans are believed to have crossed into South Africa to find work, with unemployment running at over 80 percent in their homeland and the inflation rate now more than 165,000 percent.

Mbeki's comments on his last trip to Harare four weeks ago that there was no crisis in Zimbabwe were widely ridiculed and directly contradicted by his own African National Congress party.

In a speech near Johannesburg on Friday, new ANC leader Jacob Zuma again characterised the situation in Zimbabwe as a "crisis" and implicitly rebuked Mbeki by saying it was important all sides were engaged in the mediation.

"We need to work with all parties in Zimbabwe to support the process of resolving the impasse," said Zuma, who is Mbeki's likely successor.