Greek riot police have fired tear gas to try to disperse protesters on the island of Lesbos who tried to topple a police bus during a demonstration against a European Union migration policy.

The protesters were among 2,500 demonstrators who gathered in Lesbos’s main port as Prime Minister Alexis Tsipras arrived to speak at a conference.

A large contingent of riot police formed a cordon to block the protesters from advancing further, and scores of them then tried to push over a police bus.

Officers fired tear gas and earlier discharged a flash grenade. No one has been injured or arrested. The crowd has regrouped but there has not been any further violence.

A general strike on Thursday virtually shut down the island in protest at a 2016 agreement between the EU and Turkey that has left thousands of asylum seekers stranded on Lesbos.

Greece Migrants
A man walks along an empty street in the town of Mytilene (Petros Giannakouris/AP)

Under the deal, migrants arriving on Greek islands from Turkey are held and face deportation to Turkey unless they successfully apply for asylum in Greece. The deal has created a massive backlog, angering people on Lesbos and other Greek islands.

Most businesses, shops, cafes and local government offices were shut in the main town of Mytilene. The only sign of normality was that police officers were still on duty, and the local tax office was open.

Stores were also closed in protest on the nearby island of Chios.

More than 15,000 migrants and refugees remain stuck on Lesbos, Chios and three other islands, most staying in severely overcrowded camps.

“Thousands of people are still living in appalling conditions with limited access to medical facilities,” the aid group Doctors Without Borders said.

Conditions at the largest refugee camp in Lesbos “were putting the health and lives of people stranded on the island at risk,” a spokesman said.

Additional police officers, including anti-riot units, were sent to Lesbos and took up positions around Mytilene. Supporters and opponents of the government used vans fitted with loudspeakers to promote the protest as well as a speech planned by the prime minister.

Tension on the islands has been building as the number of migrants arriving from Turkey has risen sharply in recent weeks.

“The situation on the island is exceptionally difficult. We are feeling the effects of a long-term financial crisis and the way the refugee crisis has been handled,” Lesbos Mayor Spyros Galinos said.

“For there to be any talk of growth or recovery, we must first be lifted out of this emergency situation.”

The government has promised to move thousands of asylum seekers to the mainland but says the effort will take several months to implement — requiring additional staff and more shelter sites around Greece.

In the meantime, the number of daily arrivals continues to rise on the island and Greece’s land border with Turkey.

Authorities said 53 people believed to be from Iraq and Syria were picked up on Thursday after a yacht used to smuggle them into the country ran aground on a remote mainland beach in north-eastern Greece.

The Turkish-flagged yacht ran aground on Molyvoti beach in the north-eastern Rodopi region, far from any of the usual smuggling routes used to ferry migrants and refugees from Turkey to Greece.

The group was made up of 19 men, 11 women and 23 children between the ages of two and eight.