UPDATE 1-Israel moves to block pro-Palestinian "fly-in"
* Israel says it will deport pro-Palestinian activists
* Hundreds of police deployed at Tel Aviv airport (Adds details on Brussels, Paris protest)
JERUSALEM, April 15 (Reuters) - A pro-Palestinian "fly-in" to Tel Aviv got off to a slow start on Sunday after Israel scrambled to block activists from boarding flights in Europe.
The pre-emptive steps overseas and the deployment of hundreds of police at Ben-Gurion airport to stop activists who slipped through showed Israel's deep concern over international campaigns against its policy towards the Palestinians.
Israeli police spokesman Micky Rosenfeld said that by mid-day, six passengers were detained for questioning on arrival at Tel Aviv's Ben-Gurion airport. Three were ordered deported and the other three were allowed through.
An Interior Ministry spokeswoman said the Immigration Authority had on Wednesday given airlines the names of some 1,200 activists whose entrance to Israel would be barred. Israel made clear the airlines would have to shoulder the costs of sending any deported activists back to their port of origin.
Leehee Rothschild, a "Welcome to Palestine" activist, said dozens of campaigners had since been informed by airlines that their tickets to Tel Aviv have been cancelled.
Organisers said some 1,200 Palestinian supporters throughout Europe had bought plane tickets to Israel and had planned to travel on to the occupied West Bank, an hour's drive from Tel Aviv, as part of a campaign called "Welcome to Palestine".
The aim of the so-called "flytilla", organisers said, was to help open an international school and a museum in Bethlehem.
But Israel, which described the fly-in as a misguided protest against "the Middle East's sole democracy", denounced the activists as provocateurs and said it would deny entry to anyone who threatened public order.
In Brussels Zaventem airport, around 100 Belgian and French pro-Palestinian activists were not allowed to board flights to Israel.
The activists, some of whom said they wanted to build a new school, held up letters that were handed to them at the airport which said that they were on a no-fly list because they intended to "disrupt the order and confront security forces at friction points".
Cellphone video uploaded by an activist to the internet showed about 20 pro-Palestinian activists at Charles de Gaulle airport in Paris, surrounded by police.
The group staged a sit-in at an airline check-in counter, where they chanted in French: "Let us go to Palestine".
Some Israeli political commentators said Israeli authorities had over-reacted in their response to the fly-in, playing into the hands of pro-Palestinian campaigners seeking publicity.
"We are expecting hundreds of activists throughout Sunday. Some will be sent back to their countries. As part of normal procedure, they will be questioned and each case will be decided upon individually," Rosenfeld said.
A similar, though smaller "fly-in" last year led to a few hundred activists being blocked at European airports and more than 100 others were deported after Israel denied them entry.
"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," Rothschild said.
Palestinians hope to establish a state in the West Bank and East Jerusalem, areas Israel captured in the 1967 Middle East War, and the Gaza Strip that is ruled by Islamist Hamas.
The office of Israeli Prime Minister Benjamin Netanyahu released a letter on Saturday which it hoped to hand the activists upon their arrival.
Echoing the "thank you for choosing our airline" announcements cabin crew often make to passengers after landing, the letter said: "We appreciate your choosing to make Israel the object of your humanitarian concerns."
The letter said the activists' campaign was misdirected and they could have chosen instead "to protest the Syrian regime's daily savagery against its own people".
Israel's left-wing Haaretz newspaper, criticising the government's ban, said it should invite "peace activists to visit anywhere and welcome them with flowers". (Additional reporting Claire Davenport in Brussels; Editing by Jon Hemming)