Forty of Africa's most prominent figures have published an open letter calling for Zimbabwe's presidential run-off to be peaceful and fair.
The warning came as Botswana became the first of Zimbabwe's neighbours to protest against the detention of two senior Zimbabwean opposition leaders.
President Robert Mugabe wants to extend his 28-year rule in the 27 June poll.
Mr Mugabe has said he has dissuaded war veterans from fighting to keep the opposition from power.
Political violence
The group of African leaders, which includes former UN chief Kofi Annan and Archbishop Desmond Tutu, urged an end to violence and intimidation ahead of the presidential run-off.
In the first round in March, Movement for Democratic Change (MDC) leader Morgan Tsvangirai gained more votes than Mr Mugabe, but not enough for an outright victory.
"They came to my office after the elections and asked me: 'Can we take up arms?' "
Former leaders like Ghana's Jerry Rawlings, Mozambique's Joaquim Chissano and Nigeria's Abdulsalami Alhaji Abubakar also put their names to the letter.
Mr Mugabe is still hailed by many African leaders as a hero of the fight against colonialism.
The government of neighbouring Botswana has summoned the Zimbabwean high commissioner to protest against the detention of Mr Tsvangirai and MDC Secretary General Tendai Biti on Thursday.
Botswana's Foreign Minister Phandu Sekeleman told the BBC that the arrests amounted to harassment.
"These repeated arrests do not augur well for a free, fair and democratic election - people must be free to campaign," he told the BBC's Focus on Africa programme.
He said Botswana had been affected by all the Zimbabweans fleeing the violence.
Correspondents say Botswana is the first African government to lodge a formal protest with the Zimbabwe government about the violence and intimidation that has hit the country ahead of the election.
Veteran threat
On Friday, MDC lawyers were still trying to get access to Mr Biti, who spent the night in custody after his arrest at Harare airport.
The authorities say Mr Biti faces a charge of treason and another charge for proclaiming victory in the 29 March elections before official results had been published.
A high court judge has ordered police to bring Mr Biti to court on Saturday and explain why he is being held, opposition lawyers said.
On Friday, Mr Mugabe warned that veterans from Zimbabwe's 1970s war of independence were ready to fight to stop the MDC gaining power.
The Herald newspaper quoted Mr Mugabe as saying: "They came to my office after the [first round of] elections and asked me: 'Can we take up arms?'
"They said this country was won by the barrel of the gun and should we let it go at the stroke of a pen? Should one just write an X and then the country goes just like that?"
'White oppressors'
The 84-year-old leader of the Zanu-PF party reportedly made the comments on Thursday at a rally in Murehwa, north-east of the capital Harare.
He said the MDC would hand back Zimbabwe to "our former oppressors, the whites".
The opposition, rights groups and some Western governments accuse Mugabe supporters of directing a campaign of violence and intimidation against the MDC.
The opposition says at least 66 of its supporters have been killed and some 25,000 forced to flee their homes.
The BBC has uncovered evidence that the army is behind the violence, although this has been denied by the military.
On Friday, the MDC said police had impounded two opposition election buses in Gweru, central Zimbabwe.
Mr Tsvangirai has been detained several times on the campaign trail.
Earlier, top UN humanitarian official John Holmes told the Security Council that up to four million Zimbabweans - a third of the population - needed aid.
But the UN's World Food Programme told the BBC last week 600,000 Zimbabweans currently needed aid and that figure might rise to four million early next year in a worst-case scenario.
Correspondents say that within the Security Council, South Africa - backed by Russia and China - is resisting action on Zimbabwe.
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