Israel seized 20 pro-Palestinian activists who landed at its main airport on Friday and thwarted hundreds more by threatening airlines with the cost of shipping back unwanted passengers.

Police said the 20 activists who made it to Ben Gurion Airport, near Tel Aviv, arrived from Switzerland, to be detained by security forces who were expected to expel them.

An Alitalia flight from Rome had also been diverted away from the main terminal and its passengers were undergoing "security checks," police said.

Organisers of the "Welcome to Palestine" campaign, which some have called the "flytilla," had said up to 800 activists were to fly in to Ben Gurion Airport, near Tel Aviv, in a peaceful mission to visit Palestinian families.

But Israeli authorities said they largely managed to preempt the campaign by foreign activists -- most of them from France -- demonstrating for the right of access to the West Bank.

Officials said that by notifying foreign airlines of ticket-holders who would not be admitted to Israel they prevented hundreds from boarding at their port of departure.

A statement from the public security ministry quoted regional police chief Benzi Sau saying that a joint operation by police, the foreign ministry and transport officials "prevented the departure of hundreds of activists at their points of departure for Israel."

"Public Security Minister (Yitzhak) Aharonovitch instructed that... forces continue to deploy so as to be able to deal with incoming flights this afternoon, including the prevention of disturbances, while exercising the necessary sensitivity," it added.

Earlier, immigration spokeswoman Sabine Hadad told AFP: "Israel has given airlines a list of 342 unwanted people, warning them that they will be immediately turned back at the expense of the companies."

Following the warning, "the companies have already refused to take on board around 200 of these passengers," she said, adding that two US activists who arrived were sent back to the United States.

Referring to the 20 from Switzerland, police spokeswoman Luba Samri told AFP: "They were questioned and didn't pass the questioning."

She added: "They are being detained at the moment and will probably be sent back where they came from."

Police spokesman Micky Rosenfeld said six Israelis had been arrested in the arrivals hall, which is open to the public.

"Six left-wing activists were arrested at the airport after causing a disturbance in the terminal," he told AFP. "They arrived from inside Israel, they didn't come off the planes."

Apart from deploying police in and around Ben Gurion Airport, the main Israeli tactic was to discourage airlines from allowing them to board at their points of departure.

Some 50 airline passengers who described themselves as "pro-Palestinian" were prevented from embarking on a flight to Israel from Geneva airport on Friday, officials said, prompting flight delays.

Several people belonging to a French group of pro-Palestinians had tried to get past airport security doors, resulting in authorities temporarily shutting down boarding areas.

At Roissy airport in France, at least nine activists were prevented from boarding a flight of Hungarian carrier Malev to Tel Aviv via Budapest.

Austrian Airlines said Friday it had prevented a passenger at Vienna airport from boarding a flight to Tel Aviv, following a request by the Israeli authorities.

In a statement, the organisers of the "flytilla" campaign condemned the Israeli pressure on airlines and threatened legal action.

"We call on all airline companies not to accept such provocative, blackmailing, and illegal actions by the Israeli government," it said.

"Visitors travelling between countries have rights under international law and bilateral travel agreements," it added."Those who had reservations cancelled will exercise their right of protest including bringing legal cases in their own countries."