ISTANBUL, May 27-- Turkish police detained at least 20 people in its anti-terror operation in Istanbul on Wednesday.

The semi-official Anatolian news agency reported that the arrested people are alleged to have attacked police with stones, sticks and Molotov cocktails during May Day protests in the city.

The Istanbul police department said they used a helicopter in the raid. About 800 policemen, including riot police forces and special operations teams, raided 30 addresses in 10 districts in Istanbul.

Two long-barreled rifles, one pistol and a large number of organizational documents were confiscated during the operation.

On May 1, thousands of police manned barricades and closed streets to halt demonstrations at Taksim, a traditional rallying ground for leftists that saw weeks of unrest in 2013.

However, thousands of people defied the official ban and tried to march on Taksim Square. Police fired teargas and water cannon at hundreds of stone-throwing protesters.

A total of 203 protesters were taken into custody in Istanbul during May Day demonstrations, according to provincial governor Vasip Sahin.