An overnight sit-in on board a Lufthansa plane by 56 Iranian monarchist protesters ended peacefully at Brussels Airport on Friday after Belgian police escorted them all off the aircraft.

The unarmed group of men, women and three children refused to leave the Boeing 737 flight from Frankfurt for 16 hours in a protest against Iran's Islamic government.

"The plane is empty now... there was no violence at all," said police spokesman Olivier Vincent.

He said the protesters faced only "administrative arrest", which meant they would shortly be free to leave.

One person was forcibly escorted off the plane at the end of the protest and another was treated by doctors after collapsing upon disembarking. Several riot control vehicles had assembled near the aircraft, which was flanked by police vans.

"We want the European Union to remove the Islamic leaders from Iran," Armin Atshgar, one of the protesters, said by mobile phone during the sit-in. "We want to remove the mullahs from power."

Atshgar described himself as a member of Anjomane Padeshahi, a group advocating the restoration of Iran's royal family, which was ousted in the 1979 Islamic revolution.

As he spoke, a Belgian foreign ministry official could be heard over a mobile phone inside the aircraft promising the protesters safe passage if they agreed to disembark, but warning they faced arrest if they refused.

Police spokeswoman Els Cleemput said the protesters had European passports, except two who were carrying US passports, but they were probably all of Iranian origin.

The plane had landed at Brussels' Zaventem Airport at 2 p.m. (1300 GMT) on Thursday with 103 passengers on board. The crew and all but 59 of the passengers disembarked after the plane had docked and the pilot had disabled the controls.

"As soon as we knew there was a problem one of our officers went on board and straight away it was quite clear that they were very peaceful. They were not at all the kind of people who wanted to be violent," Cleemput said.

She said that at 3:30 a.m. (0230 GMT), the protesters were given a written ultimatum, warning that they would be arrested if they did not leave immediately.

When the group refused to go, the plane was towed away from its stand towards an area of the airfield far from the cameras of journalists.

Police then went on and persuaded the protesters to leave.

"There was just one person who resisted and we had to carry him off the plane," Cleemput said. "One person fainted when he got down the steps... but we had a doctor there and took him to an ambulance straight away so we could take care of him."

Atshgar, who said he held a Belgian passport, had vowed that he and his fellow protesters would not leave the aircraft until they had spoken to a member of the European Parliament.

"We want to say to all the world's leaders, especially from the European Union, Belgium, France, Britain, Germany and the Netherlands, 'hands off and stop supporting this Islamic rule by removing it from Iran'," Atshgar said.

The EU is negotiating to try to persuade Tehran, in return for trade and security benefits, to curtail sensitive nuclear work which could enable it to produce a bomb.