Two thousand demonstrators gathered on the Greek island of Crete on Sunday to protest against the planned destruction of Syria's chemical weapons in the eastern Mediterranean, an AFP reporter said.

Despite pouring rain at the port of Souda, crowds brandishing black flags demanded more information about the international operation.

"This is the first response of the residents of Crete against the destruction of Syrian chemical weapons," Pavlos Polakis, mayor of the nearby city of Sfakia told AFP.

"If this happens it will obliterate the island's economy, will pollute the sea and will lead the people of the Mediterranean to a grim future."

The UN-backed operation to break down the chemicals aboard a US cargo ship in the eastern Mediterranean has prompted protests in Italy as well as Greece, despite assurances that the operation carries no risk to the marine environment.

NGOs and environmental groups have questioned the safety of the operation and the lack of information being provided to nearby coastal communities.

"The UN has given assurances that nothing will fall into the sea. But will they tell us where (the residue) will ultimately end up," Greenpeace Greece executive director Nikos Charalambides told AFP last month.

Greek environmental pressure group "Archipelagos" has branded the planned destruction an "environmental crime".

Under the plan, hydrolysis systems aboard the cargo ship Cape Ray are to mix heated water and other chemicals to break down the lethal agents, resulting in a sludge equivalent to industrial toxic waste.

The Cape Ray left the United States on January 27 and it is believed to be at the southern Spanish port of Rota.

Syria has declared around 700 tonnes of the most dangerous chemicals, 500 tonnes of less dangerous precursor chemicals and around 122 tonnes of isopropanol, which can be used to make sarin gas.

The UN Security Council last year backed a US-Russian deal to destroy Syria's vast chemical arsenal, but Italy and Malta have already expressed concern over the operation's environmental impact.

Under the agreement, Syria's entire chemical arsenal should be eliminated by June 30, a deadline likely to be missed owing to the ongoing conflict.

Syria shipped out a fourth consignment of chemical weapons for destruction on February 26 but international watchdog, the Organisation for the Prohibition of Chemical Weapons, has urged swifter progress to destroy the stockpile of weapons.