The Daily Mirror compared the situation in Zimbabwe to the 1994 Rwandan genocide, when the world "stood by and watched".
"Afterwards, everyone vowed such a catastrophe should never be allowed to happen again. Now it is happening again -- and the world is failing to act."
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Britain's newspapers said that Morgan Tsvangirai could not be blamed for pulling out of Zimbabwe's presidential election but urged South Africa to turn on President Robert Mugabe.
National dailies branded Mugabe "tyrannical" in their editorials and raised comparisons with Nazi Germany dictator Adolf Hitler as they urged Zimbabwe's neighbour South Africa to wield its influence and lead world outrage at Mugabe's regime.
Tsvangirai, leader of the main opposition Movement for Democratic Change party, withdrew from the two-man second round run-off on Sunday, saying violence had made a fair vote impossible. The move all but hands victory to Mugabe, who has ruled Zimbabwe since it gained independence from Britain in 1980.
The Daily Telegraph said it was hard to see what else Tsvangirai could have done. "His candidature risked bestowing a semblance of legitimacy on the whole wretched business; and in the meantime, it was intensifying the violence," it said.
The broadsheet said peaceful change in Zimbabwe was largely dependent on South Africa, as it was when the country gained its independence. "Then, as now, a minority regime in Rhodesia was prepared to plunge the country into bloodshed rather than relinquish power."
South Africa's white minority rulers looked beyond ideological links to "act in the wider interests of humanity. "If South Africa shows a similar magnanimity today, Zimbabwe might yet become a stable and prosperous neighbour. "If not, full-scale conflict will be inevitable."
Nobody could blame Tsvangirai for pulling out, said the Daily Mail. "This sick farce of a contest was never going to end in a fair result anyway. Indeed, Mr Tsvangirai was almost suicidally brave to fight on for as long as he did," the tabloid said. "What makes this tragedy worse is the way this corrupt and murderous tyrant (Mugabe) has been allowed to get away with it.
"For years, the people of Zimbabwe suffered, while the world looked on. Now they have been cheated out of their last, best hope of peaceful, democratic change. And all Africa is the loser."
The Sun quoted Mugabe as comparing himself to Hitler but noted that the Zimbabwean president has lasted far longer in power and has not been toppled by foreign powers. "The atrocities he has inflicted on his own people have earned him a place in history's hall of shame," the tabloid said of Mugabe.
"But what is equally appalling is that this monster has been allowed to stay in power for 28 years.
"While the world summoned the will to topple other pariahs, most leaders just wring their hands over Mugabe.
"Britain and our allies must turn the screw -- hard and fast. "Enough is enough. The world has stood by watching for too long."
The Guardian said Tsvangirai's decision was a "triumph for terror". "Making that call was excruciating because it killed any lingering hope that Mr Mugabe's tyrannical rule might be ended by the ballot box," it said.
Southern African leaders have slowly begun turning against Mugabe and he needs electricity and supplies from neighbouring states, the editorial read. "A wind of change blowing through southern Africa might -- just -- still finish Mr Mugabe."
The Independent said that while nobody could blame Tsvangirai for withdrawing, the decision "still leaves a bitter taste". The daily said an opportunity had been lost, but Mugabe would be weakened by the development nonetheless.
"With its economy in free-fall, its population fleeing, and an enfeebled Robert Mugabe still in power, Zimbabwe will need all the help it can get. This is the worst time for anyone, least of all its neighbours, to disengage."
The Daily Mirror compared the situation to the 1994 Rwandan genocide, when the world "stood by and watched". "Afterwards, everyone vowed such a catastrophe should never be allowed to happen again.
Now it is happening again -- and the world is failing to act."