Thousands of union workers are marching toward Istanbul's Taksim Square in the first celebrations at the site since dozens of people died there in a May 1 gathering more than three decades ago.

Police deployed thousands of officers to monitor the event.

The government has agreed to demands of labour unions to fully open the square for celebrations for the first time since 1977 when 36 demonstrators died after a shooting triggered a stampede.

The suspects were never found, contributing to a mood of instability and conspiracy theories before a military coup in 1980.

Turkey allowed a limited number of workers to hold a memorial service at the square last year after declaring May Day a public holiday following pressure from labour unions.