Meanwhile armed riot police used batons to beat about 1,000 anti-government demonstrators who ran through the streets of Harare shouting "Down with Mugabe".

The woman was decapitated in front of her two daughters, aged 10 and 17, in the Magunje area of north-western Zimbabwe on Sunday, according to the independent Daily News.

Brandina Tadyanemhandu, 53, was the mother of an MDC activist, Tichaona Tadyanemhandu, 20, who was killed by suspected Mugabe supporters in June 2000.

"They have killed my only son in a family of eight and now they have killed my wife," said distraught Enos Tadyanemhandu, 63. "Why are they fighting us after they won the election? I will not be silenced. I will speak out against this evil."

Mr Tadyanemhandu said he reported his wife's murder to the police, who told him to "bring the suspects to the police station". Police would not confirm the killing.

The partisan actions of the police was condemned by the team of Commonwealth election observers and other monitors.

The demonstration in Harare was organised by the National Constitutional Assembly (NCA) to press for a new constitution.

The assembly says that the current constitution has entrenched Mr Mugabe's rule.

Police arrested the NCA chairman, Lovemore Madhuku, and two other officials on Monday for planning an illegal demonstration.

But determined members of the NCA, a coalition of students, church and women's groups, went ahead with the protest in Harare and other cities.

To avoid pre-emptive arrests by police, the demonstrators joined the many queues for staple foods that snake through Harare. At just past noon, they left the queues to run, chanting and singing, through the main streets with the police in pursuit.

It was the first example of strategic planning to get around the police since new laws granted them sweeping powers to outlaw any demonstration or gathering.

The violence against the MDC has left 54 dead since January, according to the Human Rights Forum. Some 69,000 Zimbabweans have fled their homes and are now in hiding across the country, according to the Amani Trust human rights group.