(Reuters) - Istanbul went into a security lockdown on Friday as thousands of police manned barricades and closed streets to stop May Day rallies at Taksim Square, a symbolic point for protests.

Citing security concerns, authorities shut down much of the city's public transport and dispatched riot police to block Taksim off from demonstrators. A traditional rallying ground for leftists, the central square saw weeks of unrest in 2013.

Hundreds of flag-waving protesters gathered in the nearby Besiktas neighbourhood, where they were held back by lines of police. Thousands of protesters also gathered to march in the capital Ankara.

Critics say President Tayyip Erdogan and the government have become more authoritarian ahead of June elections.

"This meeting is peaceful and is not armed," opposition politician Mahmut Tanal, holding a pocket-sized book of the Turkish constitution, told Reuters in Istanbul.

"People want to express their problems but the government doesn't want those problems to be heard ahead of elections."

The normally thronged Isitklal shopping avenue leading to Taksim was deserted, with shops shuttered and metal barricades blocking off sidestreets. Police helicopters circled overhead.

A usually bustling square lined with cafes and hotels, Taksim was filled with police buses, ambulances and satellite broadcast trucks. A pair of tourists emerged from a hotel to find the area sealed off and nervously made their way around police lines.

The government had said Taksim would only be open to those who came peacefully and not for "illegal demonstrations".

"I wish May 1 to be celebrated in a festive mood without provocations," Erdogan said in a statement.

Opposition parties and unions called on the government to lift the ban.

Erdogan has previously dismissed protesters as "riff-raff" and terrorists, outraged by the unrest in 2013 that brought unwanted international attention and posed the biggest challenge to his AK Party since it came to power in 2002.

He is aiming for a massive victory for the party in June parliamentary polls, which would allow it to change the constitution and give him broad presidential powers.

The 2013 Taksim protests began as a peaceful demonstration against plans to redevelop Gezi Park, a leafy corner of the square. After a police crackdown the demonstration spiralled into weeks of nationwide protests against Erdogan's rule.