LONDON (AP) _ Police at English Channel ports have mounted a huge security operation after reportedly discovering a terrorist plot to bomb a ferry as it was crossing between Britain and the European continent.

In an alert which began Friday, armed police reinforcements have been deployed at British ferry and hovercraft ports and warnings sent to law enforcement officials in France, Belgium and the Netherlands, Kent county police Chief Inspector Lee Plummer said.

Police sources who spoke on conditon they not be identified said it was suspected that a terrorist group with Middle Eastern ties planned to plant explosives aboard a ferry plying the English Channel.

''There is a possibility terrorist attention may be focused on a vessel operating in either direction between this country and continental ports,'' Plummer told The Associated Press. ''We are acting against the background of recent international events.''

Britain's Independent Radio News reported that Scotland Yard's Special Branch had ''uncovered an Arab plot to blow up and sink a passenger ferry by driving a car packed with explosives on to a ferry and detonate it at sea.''

The Special Branch at the London police headquarters deals with political subversion.

Referring to the April 15 strike by U.S. warplanes at Libyan targets, the radio said the plot was ''hatched by a Middle East terror group as a reprisal for the bombing of Libya.''

Some U.S. planes used in the raid were based in Britain, and Prime Minister Margaret Thatcher approved their use for the attack. The Reagan administration said the raid was intended to punish Libya for the alleged support it gave terrorists.

Police declined comment on the radio report.

Police equipped with dogs and explosive detection equipment scanned hundreds of vehicles on Sunday before they were allowed to board the hovercraft and ferries that carry thousands of vacationers as well as big transport trucks.

Kent police set up an operations control room at county headquarters in Maidstone to coordinate the alert and keep in touch with French, Dutch and Belgian forces, Plummer said.

Kent county includes four major English Channel ports, Dover, Folkstone, Ramsgate and Sheerness, from where ferries and hovercraft depart for the European continent at the rate of more than 300 sailings a day.

''This operation will continue for at least several days,'' said Plummer. ''The last thing we want is for people to panic. But there may be delays and individual passengers may be checked.''