A May Day protester throws a stone at riot police as he and others try to break through barricades to reach the city's main square in central Istanbul on May 1.

Turkish riot police clashed with thousands of May Day protesters in Istanbul on Wednesday, firing water cannon and tear gas at crowds that tried to break through barricades to reach the city's main square, witnesses said. The incidents followed the pattern of recent years, when May Day demonstrations in Turkey's largest city have often been marked by clashes between police and protesters. Reuters/Murad Sezer