By Mark Doyle BBC West Africa correspondent

Ahead of presidential elections on 14 May, Mark Doyle keeps a diary from the campaign trail.

Day 4 - Violence in Freetown

A bare chested man with blood pouring from his head is fighting to get into my car. United Nations soldiers are shooting in the air and police sirens are blaring. Volleys of stones are flying up and down Lightfoot Boston street in central Freetown.

The election campaign in Sierra Leone has started to go sour.

Just up the road, an SLPP supporter was being lynched, he only survived being stabbed with a knife because a policeman grabbed the assailant's hand

Until Saturday campaigning the length and breadth of Sierra Leone had been going remarkably peacefully for a country that just two years ago was engulfed in all-out war.

But then the rocks started flying.

The riots broke out between supporters of the ruling party, the Sierra Leone People's Party (SLPP) when they tried to march past the offices of the former rebels of the Revolutionary United Front Party (RUFP).

Both sides blame the other for starting the stone-throwing - with the RUFP adding that their offices were trashed and looted by SLPP.

Warning shots

What is not in dispute is that for a few hours on Saturday lunchtime central Freetown was out of control.

The Sierra Leone police were there, apparently trying to keep the two sides apart but unable to do so. Then Nigerian, Pakistani and other UN peacekeepers arrived on the scene in their big white armoured personnel carriers, firing warning shots into the air.

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As I watched this scene from what I thought was a safe distance, a badly injured RUFP man, with blood streaming from his head, tore open the door of my car and tried to commandeer it to take him and another injured man to hospital.

"The SLPP did this to me! I want the BBC to say that!" he shouted as he climbed uninvited into my car.

Just up the road, an SLPP supporter was being lynched. He only survived being stabbed with a knife because a policeman grabbed the assailant's hand.

I refused to allow the RUFP man into my car because of the danger of my becoming embroiled in the action by having a participant in my car.

Then, thankfully, a policeman rescued me just like that other officer had saved the SLPP man. The man in a smart blue uniform yanked the former rebel off my running board and advised me to leave the scene.

Permission confusion

At this point I decided - partly because I have a broken ankle from an accident in the Ivory Coast rainforest, and am not very mobile - that discretion was the better part of valour.

Questions have to be asked about why the two groups were scheduled to hold rallies, of whatever sort, on the same day

I retreated in my hire car to the headquarters of the police Criminal Investigation Department (CID) on the waterfront.

Officers there told me that the trouble appeared to have been caused by the RUFP holding a larger gathering than they had had permission for, and that this led to a confrontation.

The RUFP, according to the CID, had been given permission for a women�s march but not a major rally, and certainly not one involving the hundreds of young RUFP men I saw on the streets.

At the same time, thousands, possibly tens of thousands, of supporters of the ruling SLPP were converging on the National Stadium for their last campaign rally. The route of some of the SLPP backers passed the Lightfoot Boston Street HQ of the RUFP.

Who threw the first stone is of course still in dispute. But questions have to be asked about why the two groups were scheduled to hold rallies, of whatever sort, on the same day.

The incident was - so far - an isolated one in a generally peaceful campaign.

But it left over a dozen people seriously injured and has soured the atmosphere ahead of Tuesday�s election.