President Thabo Mbeki met Wednesday in Zimbabwe with longtime leader Robert Mugabe amid mounting concern about political violence in the neighboring nation.

Also Wednesday, the Zimbabwean opposition's No. 2 leader was brought to court — but the hearing was abandoned because of a power failure. Tendai Biti was expected to be charged with treason at another hearing set for Thursday, his lawyer said.

Mbeki's trip to Bulawayo, Zimbabwe's second main city, was part of his efforts to mediate between Mugabe and opposition leader Morgan Tsvangirai, who face off in a presidential runoff June 27, the Foreign Affairs Ministry said in a statement.

Reporters saw Mbeki arrive at the Bulawayo airport and then head to a hotel. Mugabe, who had been campaigning in the region, arrived at the hotel later.

South African officials gave no more details about the trip, which was first reported Wednesday in The Herald, a Zimbabwean government mouthpiece.

Mugabe has come under increasing criticism, accused of a campaign of violence against the opposition aimed at ensuring he wins the runoff after coming in second to Tsvangirai in March. But Mbeki has steadfastly refused to publicly criticize Mugabe, saying confrontation could backfire.

Tsvangirai's Movement for Democratic Change party expects little from Mbeki's trip, and none of its officials plan to meet with the South African leader, party spokesman Nqobizitha Mlilo said. Tsvangirai has called on Mbeki to step down as mediator, accusing him of bias toward Mugabe.

"We don't even know what his (Mbeki's) agenda is," Mlilo said. "We can't attach expectations to an agenda we don't know anything about."

In a speech before parliament last week, Mbeki said his priority was to help Zimbabwe find answers through dialogue and negotiation rather than imposing a solution from abroad. That has been his stance for more than a year, since other regional leaders appointed him to mediate in Zimbabwe's crisis.

Other African leaders, though, have said quiet diplomacy is failing. Kenya's Prime Minister Raila Odinga, addressing reporters in Washington on Tuesday, urged world leaders to pressure Mugabe to step down, and called the runoff a "complete sham." Jacob Zuma, head of Mbeki's African National Congress, was quoted by South African media Wednesday as saying the runoff was unlikely to be free.

In addition to the violence, the opposition's campaign has been repeatedly interrupted by police road blocks that have often ended with Tsvangirai spending hours at police stations. The opposition says the June 12 arrest of Biti, secretary general of Tsvangirai's party, was another attempt to disrupt its campaign, and leaders dismiss possible treason charges as politically motivated.

Biti was brought to court in leg irons Wednesday but because the lack of electricity meant recording equipment could not work, both the prosecution and defense lawyers agreed to postpone the hearing until Thursday, opposition lawyer Lewis Uriri said. Uriri said he expected Biti to be formally charged with treason, which can carry the death penalty, and other counts.

In Geneva, U.N. Human Rights Chief Louise Arbour told reporters her staff member went to the Zimbabwean capital, Harare, on Sunday to meet with U.N. workers in Zimbabwe and help aid groups there.

Arbour says the official was expelled Tuesday — the same day a U.N. special envoy met with Mugabe. She did not give a reason for the expulsion or identify the official.

Mugabe's government recently ordered humanitarian groups to suspend work in the country, accusing them of helping his opponents. Several aid groups said Wednesday the government ban on their work had been partially lifted last week.

However, Fambai Ngirande of the National Association of Non-Governmental Organizations said providing help in rural areas remained "very dangerous."

U.N. Secretary-General Ban Ki-moon sent envoy Haile Menkerios, who met with Mugabe on Tuesday, "for discussions on the political situation and the upcoming elections," according to Ban's office.

The Herald played down Menkerios' visit, calling it part of a tour of the region.

"The two powers are now banking on Mr. Menkerios to deliver a verdict that will boost their regime change agenda," the editorial said. "It is against this background Zimbabweans will not receive the U.N. official with open arms."