Strikers also briefly blockaded the entrance to the Channel Tunnel

But hundreds of lorries remain backed up on motorways in both Dover and Calais.

Seamen blocked Calais port on Thursday, a day after fishermen halted services with protests over EC quotas.

P&O and SeaFrance have both begun running services and hope to clear all passengers by Friday afternoon.

One P&O Ferries manager in Calais said he had not seen such a heavy backlog of traffic in his 30 years of service.

'Contempt'

Police in Kent have closed part of the M20 to allow lorries to queue on the motorway for the next ferry.

The company said tens of thousands of tourists and lorry drivers had been affected by the blockade on Wednesday, and Thursday's strike by 1,200 SeaFrance staff over proposed changes to employment law.

We can only apologise to passengers who have been affected SeaFrance Backlog of lorries after port action

P&O's first ferry left Dover at 0530 GMT on Friday, arriving in Calais shortly after the strike ended at 0700 GMT.

A spokesman said the huge capacity of its vessels meant the backlog could be cleared before the end of the day.

SeaFrance said its first service left Calais at 0900 GMT and would depart from Dover again at 1100 GMT.

"We can only apologise to passengers who have been affected," said a spokesman.

P&O said it had "respect" for the French workers' right to strike, but had "complete contempt" for the nature of the action.

Stranded

Their spokesman said an "ugly mob" of around 200 strikers threw bottles and luggage trolleys into the water to prevent a P&O ferry trying to dock in Calais on Thursday.

The company succeeded in getting a court injunction against the activists, but said the French authorities did not enforce it.

In a further attempt to break the blockade the chairman of P&O met with the French ambassador in London.

Among those stranded on Thursday were hundreds of schoolchildren trying to return from trips to France.

Sailings to Cherbourg were also affected with Brittany Ferries forced to cancel a service from Poole.

French unions say the plan to liberalise merchant shipping threatens jobs and safety.