Army spokesman Lieutenant Colonel Robin Hodges said the suspect, who has not been identified, was a member of the Serbian Interior Ministry police.

The arrest came amid growing concern about violence in the province, with some returning Kosovo Albanians determined to seek revenge on Serbs.

Duncan Kennedy reports on the symbolism of the arrivals at Pristina airport

On Saturday an advance party of Russian peacekeepers flew into Pristina, the first of a 3,000-strong force. More troops are expected on Monday.

Pristina airport had been closed since about 200 Russian soldiers beat Nato forces into Kosovo two weeks ago, provoking a stand-off between the two sides.

The arrival of the Russian plane broke the airport deadlock

The war crimes suspect was arrested at his home in Lipijan, about 10 miles south of the capital Pristina.

The man - who is not on a list of suspects indicted by the international war crimes tribunal - was arrested after a number of witnesses gave statements, saying they had seen some of the murders.

Lieut Col Hodges said: "He is being held in detention by the British Army and the expectation is that he will be held until we receive instructions from the international criminal tribunal.

The BBC's Michael Voss: Ethnic Albanians are nervous

"We do not believe the 56 murders were carried out at the same time. We think they were a series of incidents."

British forces had been given evidence after entering the area on 16 June.

Angry march

Tensions between ethnic groups in Kosovo continue to mount.

Jeremy Bowen in Kosovo: Albanians have looted and torched Mitrovica

In Mitrovica, Kosovo Albanian leader Hashim Thaci defused a potentially violent confrontation on Saturday when he stopped hundreds of people marching into the Serb-held side of the town.

Mr Thaci, along with firm action by French troops, persuaded the crowds to retreat. "We want to solve our problems peacefully," he said.

He was accompanied by the UN special envoy for Kosovo, Sergio Vieiria de Mello, who said the march "would be a disaster" if it proceeded.

The UNHCR has said it will organise the full-scale return of refugees to Kosovo from Monday.

(Click here to see a map showing refugee movements)

Ron Redmond, UNHCR spokesman in Pristina, said that 10 buses would run from two camps in Macedonia, with buses from camps in Albania planned for the following days.

Meanwhile in Yugoslavia, the state of war imposed three months ago was lifted on Saturday, but opposition politicians feared that some restrictions would continue.

Hundreds of Yugoslav army reservists who served in Kosovo during the Nato bombing campaign ended protests in a number of towns in central Serbia.

The reservists removed armoured vehicles they have been using to block roads and bridges in a protest over wages, after the intervention of senior army officers.

In Montenegro, the republic which, with Serbia, makes up Yugoslavia, the army has begun dismantling roadblocks and demobilising reserve forces.

• Russians arrive in Kosovo
• Coup fears as Montenegro demobilises
• Reward for Milosevic capture
• Eyewitness: When society breaks down
• Kosovo Kid replaces bullets with fists