Zimbabwe opposition leader Morgan Tsvangirai has left hospital in a wheelchair after allegedly being beaten in police custody on Sunday.
Four of Mr Tsvangirai's colleagues remain in hospital
He told the BBC that the government had miscalculated, as it was seen more than ever as a "rogue regime".
In the UK, Foreign Office minister Lord David Triesman told the BBC Zimbabwe's government had committed "actions... bordering on crimes against humanity".
Zimbabwean leader Robert Mugabe has said Western critics should "go hang".
He blamed the violence on Mr Tsvangirai's Movement for Democratic Change (MDC).
The two MDC factions have meanwhile announced that they will now work together.
Their 2005 split is widely believed to have greatly weakened their struggle against Mr Mugabe.
The two MDC leaders shared the platform for the first time at Sunday's rally and both men were subsequently arrested and allegedly assaulted.
"We let down Zimbabweans in 2006 by bickering and name-calling among ourselves," said Tendai Biti, a close ally of Mr Tsvangirai.
Mr Tsvangirai strongly denied being involved in any violence and urged Africa and especially South Africa to increase pressure on President Mugabe.
UK Foreign Secretary Margaret Beckett has said she holds Mr Mugabe personally responsible for Mr Tsvangirai's injuries.
"He is in charge of the government, he's made it very clear that this is a deliberate act of policy on behalf of the government of Zimbabwe, he is indifferent I think to the real horror that's felt right across the international community," she told the BBC's Politics Show.
TSVANGIRAI'S LEGAL TROUBLES
2003: Charged with treason - later dropped
2002: Lost election to Mugabe, charged with treason - later dropped
2000: Charged with treason - later dropped
2000: MDC won 57 parliamentary seats
1999: Helped form MDC
Lord Triesman pointed to the government's suppression of the MDC and the eviction of hundreds of thousands of people from urban areas in 2005 as possible crimes against humanity.
He dismissed calls for military intervention in Zimbabwe but supported calls for those responsible for assaulting Mr Tsvangirai and some 50 other MDC activists to be identified and added to the list of those subject to an EU travel ban and assets freeze.
The US says it is also considering extending its sanctions on Zimbabwean officials.
Mr Tsvangirai was arrested together with colleagues after police broke up a banned rally on Sunday. He appeared in court two days later with a badly bruised face and stitches in a head wound.
However, doctors deny reports that he suffered a fractured skull and say he has suffered no permanent brain or scalp damage.
His spokesman, William Bango, said Mr Tsvangirai was still in pain.
Four other MDC officials remain in hospital.
Speaking after a meeting with Tanzanian President Jakaya Kikwete on Thursday, Mr Mugabe said there had been no Western criticism of the MDC, which he said had instigated the violence.
"When they criticise the government when it tries to prevent violence and punish perpetrators of that violence we take the position that they can go hang," he said.
More than 80% of Zimbabweans are living in poverty, with chronic unemployment and inflation running at more than 1,700% - the highest in the world.