Zimbabwe MDC politician Sten Zvorwadza beaten in Mbare
A Zimbabwean politician has been attacked in the capital, Harare, during the last day of campaigning before a referendum on a new constitution.
Sten Zvorwadza, who hopes to become the next Movement for Democratic Change MP for the city's Mbare suburb, was punched as he tried to put up posters.
He escaped uninjured and says the youths were almost certainly Zanu-PF supporters of President Robert Mugabe.
Both Zanu-PF and MDC are campaigning for a "Yes" vote in the referendum.
But the referendum campaign has been raising political tensions ahead of an election expected later in the year.
The parties have been in a coalition government for the last four years following a disputed vote in 2008 marred by violence.
The BBC's Andrew Harding witnessed the incident in Mbare - an MDC stronghold in Harare - and said the youths also threw punches at him and his film crew.
Although no-one was seriously injured, the incident is a reminder of the violence lurking close to the surface in Zimbabwe, he says.
Under the new constitution, the president who wins the election, expected to be held in July, will be able to serve a maximum of two terms.
Mr Mugabe and MDC leader Morgan Tsvangirai, who is currently serving as prime minister, are both expected to vie for the presidency again.
Mr Mugabe, 89, has been in power since independence in 1980.
On Wednesday, he expressed concern over reports of a resurgence in political violence across the country.
His comments came after a Zanu-PF official was injured on Tuesday after his house was petrol-bombed by unknown assailants in Makoni district, in north-eastern Zimbabwe.
In February, the 12-year-old son of an MDC activist died in an alleged arson attack in the eastern farming district of Headlands.
Zanu-PF spokesman Rugare Gumbo said the president wanted people to vote peacefully.
"He said we should not have violence and added that the party will not tolerate that nonsense whether it is intra-party or inter-party. He wants the people to vote in peace," Zimbabwe's state-run Herald newspaper quotes Mr Gumbo as saying.
'Midwife' to new Zimbabwe
Analysts say the constitution is seen as a compromise document.
During campaigning the MDC has focussed on clauses guaranteeing protection against all forms of violence, and free and fair voting.
The MDC's Tendai Biti, who currently serves as finance minister, told the BBC the document was the "midwife" to a brand new Zimbabwe as it sets out people's rights, devolves some power and sets up a system of check and balances for those in authority.
The Zanu-PF campaign has highlighted the irreversibility of the land reform programme, which saw some 4,500 farms seized from mainly white commercial farmers, and other moves intended to give more economic power to black Zimbabweans.
Western and US observers have been barred from the referendum, but some 2,000 local and other foreign observers have been accredited for Saturday's vote, the Herald reports.
The Zimbabwe Election Support Network (Zesn), a network of 31 non-government organisations whose head office was raided by police in February, is deploying about 600 observers.
It said in a statement on Friday that it was worried about new regulations from the electoral commission limiting the number of observers at polling stations.
It was also concerned by the refusal to accredit Jestina Mukoko, director of the Zimbabwe Peace Project, Zesn said.
Ms Mukoko spent two days in police custody earlier this month being questioned on allegations of illegally distributing radios.
Over the last four months, several hundred shortwave radios have been confiscated by the authorities, who say that have been illegally donated by Western organisations.
Many people in rural areas get most of their news from radio, which is dominated by the state broadcaster, although several stations based abroad broadcast to Zimbabwe on shortwave.