By Pierre Savary

LILLE, France (Reuters) - A fire in the Channel Tunnel was put out on Friday almost 20 hours after it took hold on a freight train, and the tunnel operator said passenger services between Britain and France should resume slowly over the weekend.

An official said there were indications the fire had started by accident but it was too early to identify the exact cause.

Eurotunnel, which manages the undersea rail link, said its technicians were checking all the security systems of one of the tunnels, which had not been affected, in the hope of resuming "commercial traffic" soon.

"We will be able to resume traffic tonight," Eurotunnel Chief Executive Jacques Gounon told French TV station TF1.

Gounon said two empty shuttle trains would travel before midnight. The company hoped to let freight trains through between 1200-0100 GMT while passenger trains were expected to resume service later on Saturday morning.

Eurostar, which operates the cross-Channel trains, had told passengers earlier that it did not expect to be back running trains on Friday. Anyone holding tickets for weekend trains should consult its www.eurostar.com website, it said.

No one was killed in the fire, which turned one of the two tunnel shafts into an inferno, with temperatures reaching 1,000 Celsius (1,832 Fahrenheit).

This section of track might take weeks to repair, but Eurotunnel's chief executive Jacques Gounon told French radio the adjoining tunnel "had not suffered any damage".

About 40,000 people a day use the tunnel to travel between Britain and continental Europe and thousands of passengers were left stranded by the incident.

"I was expecting them to give us a solution, to get a train or a plane ... but it's our problem, they said," said Isadora Cruciol, an assistant manager at a hotel in England, who turned up at Paris's Gare du Nord station hoping for information.

French rail operator SNCF said traffic was expected to proceed slowly in the unaffected tunnel throughout the weekend.

TOXIC FUMES

Magistrates have opened an investigation into the fire, which officials think began on a lorry loaded on the shuttle.

French prosecutor Gerald Lesigne told a news conference initial findings pointed to an accident. "We are looking at facts which point to an accident... at this stage no indication on the origin of the start of the fire is possible," he said.

Eurotunnel's Gounon had said the blaze took hold some 40 km (25 miles) into the 51-km tunnel, towards the French end.

He said some truck drivers who had been traveling in a sealed compartment on the shuttle smashed windows to escape.

They should have waited until ventilation systems had removed toxic smoke before looking to reach the service tunnel, he added. Six people were taken to hospital after inhaling the fumes and eight others suffered cuts and bruises.

Truck drivers caught up in the blaze said they had felt trapped in the stranded train.

"The door of our carriage was locked. It was impossible to open it. We saved ourselves by breaking a window with a hammer," Belgian truck driver Patrick Lejein told Le Parisien newspaper.

"Everything was exploding around us -- tires, fuel tanks and then there was this smoke which stopped us seeing and breathing properly," he added.

Any prolonged disruption to services would be a blow for Eurotunnel, which posted its first profit only last year, but Gounon said the company was insured and he did not expect any financial problems as a result of the blaze.

Eurotunnel's Paris-listed shares closed down 0.6 percent at 8.95 euros.

Opened in 1994, the Channel Tunnel is the longest undersea subway in the world. There have been two previous blazes in the tunnel, both involving lorries being transported on trains, with a 1996 fire halting freight traffic for seven months.

(Additional reporting by Francois Murphy, Gerard Bon and Nathalie Meistermann; writing by Crispian Balmer; editing by Philippa Fletcher)