Pitched battles on the streets of Istanbul.

Turkish police used what they could to shut down May Day protesters from defying Prime Minister Tayyip Erdogan and reaching Istanbul's central Taksim square.

The Istanbul governor's office said it had received advanced information that "illegal terror organizations and their extensions" would resort to violence to stoke unrest.

But security measures failed to deter thousands from trying to march, with pockets of protesters playing cat and mouse with police in tear gas-shrouded side streets.

Some 40 people were hospitalized and around 160 detained

Erdogan has in the past dismissed protesters as "riff-raff" and "terrorists."

But he has faced criticism from the European Union which Turkey aspires to join, over restrictions on freedom of speech including a two-week ban on Twitter and last summer's police crackdown on demonstrations.