British paratroops and Gurkhas on board Chinook helicopters were the first Nato soldiers to enter the province, at 0305 GMT.

Wave after wave of Chinooks, supported by US Apache helicopters led the way, crossing into Kosovo airspace from the Former Yugoslav Republic of Macedonia.

The BBC's Rageh Omaar: "Entering Kosovo"

Beneath them, thousands of troops lined up in convoy, began crossing the ground border at Blace - the same frontier post used by thousands of Kosovo Albanian refugees as they fled President Slobodan Milosevic's programme of ethnic cleansing.

Brigadier Adrian Freer, commander of the British 5th Airborne brigade and the first Nato officer to cross the land border at Blace, said: "We're going in to secure peace and make peace."

The BBC's Paul Royall reports: "Seems the push into Kosovo has ended as a race"

The deployment is one of the biggest military operations in Europe since World War II.

It is intended to help thousands of refugees return home, though they have been warned they will have to wait until the province is made safe.

Although the international peacekeeping operations was launched with the authority of the United Nations, most of troops are from the Nato nations which carried out the air campaign against Yugoslavia.

British units are being joined by French and German forces - the advance guard of a force that will eventually number about 50,000.

French troops, led by mine clearing experts from the French Foreign Legion, moved into Kosovo along two lines of advance several kilometres apart and to the east of Blace.

Military sources said part of the advance was held up when special forces commandos detected a minefield just a few hundred metres into Kosovo.

Russian surprise

Russian troops: Mobbed in Pristina

But hours earlier, Russian troops had been given a heroes' welcome by cheering crowds of Serbs when they unexpectedly arrived in Pristina.

In a remarkable turnabout, Russian Foreign Minister Igor Ivanov immediately ordered them back out again.

Mr Ivanov called the move "unfortunate" and said they had been ordered to leave Kosovo.

He said the troops did not have government authorisation to enter Kosovo.

White House spokesman Joe Lockhart said the Russian explanation has been accepted.

(Click here to see an animated map showing timetable of Serb withdrawal and Nato's planned movements)

It is not clear how the Russian action will affect Nato's planning.

News reports from Pristina say the column of Russian armoured cars, trucks and troop transporters drove through the city centre and parked on the road to the airport.

Vital road

The BBC's Paul Wood in Kosovo: "Jubilation among KLA fighters"

Paratroopers and Gurkhas immediately began securing the landing zones and high ground above the road to Pristina.

The British Nato troops are travelling along the strategically-important road from Blace to Pristina.

Forward headquarters staff on the Kosovo border oversaw the deployment of the peace implementation force, K-For.

American forces have also been involved in the first stages of the operation to enter Kosovo from Albania and Macedonia.

Nato officials have planned for a "substantial" force to be in Kosovo by Saturday afternoon but have not released details of the operation.

In the hours before the troop movement, a British transport plane crashed on a military airfield near the Albanian town of Kukes, injuring one person.

Moscow's pledge

The BBC's Mike Williams in Pristina: "Sound of gunfire announced their arrival"

Russian forces based in Bosnia, with the UN peacekeeping force there, first crossed into Serbia on Friday.

Nato feared the Russian convoy, bearing new K-For markings, would try to reach Pristina ahead of its own troops.

British paratroops were put on stand-by to fly to Pristina to head off a possible Russian attempt to take control of the airport there.

But they were stood down after Washington received assurances from Moscow that it would not go in first.

Russia's role in K-For, a UN operation but whose manpower is predominantly from Nato nations, has not been fully determined.

Moscow and Nato have been debating Russia's role since the peace deal was signed.

US Deputy Secretary of State Strobe Talbott had left Moscow after inconclusive talks on the future of the Russian presence in Kosovo when the reports came through about the convoy arriving in Serbia on Friday morning.

He abruptly turned his plane around and went back into talks with Russian officials which have continued through the night.

Correspondents say Mr Talbott had refused any modification to the UN-backed formula for peacekeeping in Kosovo.