Jenni Williams, whose Women of Zimbabwe Arise movement (Woza) has been at the forefront of protests against Robert Mugabe's regime, denounced the power-sharing coalition as a "failure" and warned expatriate Zimbabweans not to return home.
Mr Tsvangirai, the former opposition leader, is expected to make an emotional appeal at Southwark Cathedral to the Zimbabwean diaspora living in Britain to return to the impoverished southern African nation. But Ms Williams condemned the initiative.
"How can he ask Zimbabweans to come home when his own people are being beaten for saying they are refugees in their own country?" she said in Zimbabwe.
Eleven members of Woza have been arrested and tortured since Thursday, she said. Four in Harare were released needing hospital treatment and seven were being refused bail in the western city of Bulawayo.
"These women were brutalised by Morgan Tsvangirai's police," she said.
Mr Tsvangirai's supporters argue that his faction does not control the security forces, which are still run by President Mugabe's lieutenants, and that he cannot be judged on their actions. Ms Williams said this excuse was "not good enough" from a man supposed to be the Prime Minister.
While many of her contemporaries have left the shattered country, 56-year-old Ms Williams, of Irish-Ndebele extraction, has led a grass-roots movement which has staged peaceful demonstrations throughout the worst of the Mugabe years. She has been arrested more than 30 times and, along with hundreds of Woza members, she has been regularly beaten. The movement, which has 60,000 members, focuses on non-violent protest and does not contest parliamentary elections.
Ms Williams dismissed as a "complete failure" the unity government formed by Mr Mugabe's Zanu-PF party and their former opponents, the Movement for Democratic Change.
Many of the reforms called for in the unity deal brokered by South Africa to end a post-election crisis have not been implemented, she said. "The agreement is there. It doesn't need money – what it needs is political will."
Other groups echoed Ms Williams' warning that little has changed in Zimbabwe. "I am very much afraid that Tsvangirai is being used by Mugabe as a facade to attract EU donor money and that they will do away with him and his party once they have got what they want," said Fambai Ngirande, from Zimbabwe's national association of non-government organisations.
Western countries have stopped giving Zimbabwe development aid in the past eight years as the Mugabe regime used increasingly violent tactics to stay in power. Personal sanctions such as travel bans have also been placed against many of those in Mr Mugabe's inner circle.
However, humanitarian assistance that bypasses the government has continued, despite the government's attempts to portray itself as the victim of punitive sanctions.
Mr Tsvangirai has been attempting to convey to the world that the unity government is working.
But Abel Chikomo, the director of a human rights forum in Zimbabwe, said: "There is no reason for the EU to lift their measures that ban the Zanu-PF cadres, including Mugabe, from travelling to the EU and freeze their assets until Mugabe, his party and the military abide by the rule of law and show tangible commitments to the unity government."
Barbara Stocking: Country faces yet another famine year
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The Zimbabwean Prime Minister, Morgan Tsvangirai, arrives in the UK this weekend, riding a wave of optimism about the Government of National Unity (GNU). While US President Barack Obama has committed $73m to assist in Zimbabwe's recovery he made it clear that this aid will be channelled through NGOs instead of going directly to the GNU, because of concerns about human rights and the rule of law.
It is aid Zimbabwe desperately needs. The cholera epidemic that killed more than 4,000 people and infected more than 98,000 will re-emerge in the next rainy season if that country's water and sanitation system is not fixed. The UN warns that a fresh outbreak could kill as many as 125,000.
Last year's food crisis left five million people depending on food aid. This year's harvest is predicted to be better but still not enough, and in the "hungry season" before the harvest, starvation will soar again, particularly among the most vulnerable members of the population.
An Oxfam staff member told me how, on a recent visit to Bulawayo, she had met an 80-year-old woman who had borne the brunt of the food crisis. She recounted how she had often not eaten anything for days. "Then I would just boil water and drink it while it was warm to fill my stomach. My skin was hanging off me."
She also told how the dollarisation of the economy – in which foreign currencies replaced the hyperinflationary Zimbabwe dollar – has helped to curb inflation and ensure shelves in shops are stocked. But for people who have no access to currency, particularly orphans, the elderly and people with disabilities, this has made life even harder than it was before.
The British Government must begin to look for ways to fund recovery, even if they do not yet want to channel funds directly through the government. This could be by providing technical support to particular ministries or channelling funding through the United Nations and non- governmental organisations.
For its own part, Zimbabwe's government must uphold the rule of law, repeal all repressive laws and give the space to civil society groups to engage on all fronts.
Barbara Stocking is the chief executive of Oxfam
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