ATHENS (Reuters) - Greek police fired teargas at dozens of immigrants pelting them with sticks and metal objects during a protest at a detention center in the northeastern city of Komotini on Friday, police said.

The clashes broke out after more than 400 of about 550 immigrants held at the center protested against their living conditions, shouting "Freedom!" and "Send us home!".

Some set fire to dozens of mattresses.

At least four immigrants and four police officers were injured, police said.

Greece is a gateway for mostly Asian and African migrants trying to enter the European Union. The country's deep economic crisis has fuelled tensions between Greeks and immigrants, who right groups say are often held in poor conditions or subject to racist attacks by a powerful far right.

About 130,000 immigrants cross Greece's sea and land borders every year, most via Turkey, and authorities are forced to release those arrested because of a lack of permanent housing.

Komotini, a town hard hit by the recession, is close to the Turkish border.

Police launched a sweep operation in August called "Xenios Zeus" after the ancient Greek god of guests and travelers. They have so far arrested thousands of suspected illegal immigrants.

(Reporting by Karolina Tagaris; Editing by Angus MacSwan)