The son of a Zimbabwean opposition leader has been kidnapped and beaten to death along with three other men, in the latest politically-motivated killing attributed to the Mugabe regime.

Archfred Chipiyo was one of three men discovered by the roadside in Harare today with their skulls smashed after they were abducted by loyalists of the president's Zanu-PF party on Tuesday. The body of a fourth man was later discovered nearby.

The Movement for Democratic Change (MDC) has accused President Robert Mugabe's government of waging war on its activists in an attempt to intimidate voters in next week's elections.

Today the government filed treason allegations against Tendai Biti, the MDC's number two to Morgan Tsvangirai. The papers lodged with a Harare court said that a possible penalty for being found guilty would be the death penalty.

Archfred Chipiyo was kidnapped on Tuesday night after armed security forces raided the home of his father, Suleiman, the MDC chairman in the Chinunwiza district. Suleiman Chipiyo escaped from the scene with a number of other activists, but his son and three others were left in the house, surrounded, and badly beaten up before being abducted. One of those killed was believed to have been a passer-by caught up in the attack by chance.

The MDC said that the latest killings brought to about 70 the number of its activists, their spouses and children who have been murdered by Zimbabwean security forces and government supporters since the first round of the presidential election on March 29 failed to yield a conclusive result.

Thabo Mbeki, the South African President, flew to Zimbabwe yeterday to urge Mr Mugabe to negotiate a unity government with the opposition in an attempt to control the violence ahead of next-week's presidential election re-run.

"Now it’s about 70 we’ve lost," said Nelson Chamisa, an MDC spokesman. "The situation in the country is getting worse. A free and fair election is impossible."

The MDC says that the army-led campaign of intimidation began after the first vote, in which Mr Tsvangirai defeated Mr Mugabe but fell short of an outright majority. Mr Tsvangirai has said that Zimbabwe is now run by what is essentially a "military junta".

Until the last week the violence has mainly been confined to rural areas, but the most recent attacks have taken mob action from poor townships into prosperous suburbia.

In the past week, the wives of at least three MDC officials have been murdered. Abigail Chitoro, 27, the wife of Harare's recently elected MDC mayor, was so badly beaten by the mob that dragged her and her four-year-old son from their home that even her brother-in-law struggled to identify the body. The clothes she was wearing, her distinctive haircut and the blindfold that Zanu (PF) supporters forced her to wear as they firebombed her home gave the only clue to her identity. The child was dumped alive at a police station.

The UN has blamed Mr Mugabe’s supporters for most of the attacks, but Mr Mugabe says that the violence is being led by the MDC, and has threatened to arrest more opposition leaders.

It emerged that Zimbabwe authorities had filed court papers accusing Mr Biti of "subverting a constitutional government as defined in the criminal law.” The court file also shows prosecutors opposing bail for Biti since “the accused is facing serious offences which attract capital punishment".

Mr Mbeki met Mr Mugabe and Mr Tsvangirai separately in Zimbabwe yesterday to try to mediate an end to the increasingly violent crisis.

South Africa’s Business Day newspaper, a respected financial daily, quoted unnamed sources as saying that Mr Mbeki tried to set up a meeting between the pair — their first — but did not receive a firm commitment from the Zimbabwean President.

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