About 650 police officers, some in plain clothes, patrolled Ben Gurion airport on Sunday ready "to prevent disturbances and disruption", according to police spokesman Micky Rosenfeld.
Around 50 passengers had been detained and interrogated by mid-afternoon, with many more flights expected during the early evening. Nine people were immediately deported, and a small number were allowed entry to Israel. Those suspected of being pro-Palestinian activists were taken first to a smaller terminal, with a "Welcome to Israel" sign above its doors, for interrogation and from there to a nearby prison.
At least four Israeli sympathisers who unfurled a banner reading "Welcome to Palestine" in the arrivals hall were arrested.
"The security measures are to prevent any major incidents taking place," said Rosenfeld. "These are not terrorists, but they could be a threat to Israel's security."
Israel had distributed a list of about 1,200 names of personae non grata to airlines, demanding they be barred from flights. "Failure to keep this instruction could lead to imposing sanctions against the airline," said a letter from the immigration authority. Carriers were told they would bear the cost of repatriating deported activists.
Hundreds of ticket-holders were told hours before their flight that they would not be permitted to board planes. At Brussels Zaventem airport, around 100 activists showed letters barring them from travel, which said they intended to "disrupt order and confront security forces at friction points".
Up to 1,500 activists, mostly from Europe, had been planning to travel directly from Ben Gurion airport near Tel Aviv to Bethlehem in the West Bank for a week-long programme of educational and cultural activities under the banner of "Welcome to Palestine". Israel controls all entry points to the West Bank.
Organisers accused Israel of being paranoid and hysterical. "Israel's willingness to detain people who have not committed any crime and have done nothing but say they came to visit Palestine is a hysterical reaction," activist Leehee Rothschild told Associated Press.
Activists who landed at Ben Gurion were handed a letter suggesting that humanitarian issues in Syria and Iran, and Hamas's rule in Gaza, were more worthy of protest than Israel's policies in the West Bank.