THESSALONIKI  - Twelve undocumented migrants and four policemen were injured on Friday in a violent protest that broke out inside a detention centre in Komotini, northern Greece, local police said.Scores of migrants protesting against their open-ended detention threw stones and broken masonry at the police, who fired back tear gas, the police said. Around 500 migrants, mainly from Pakistan and Afghanistan, are held at the centre, one of many temporary facilities set up around Greece in an effort to deal with the country's acute illegal immigration problem.Crisis-hit Greece has for over a decade been a prominent entry point into the European Union for hundreds of thousands of undocumented migrants and refugees from Asia and Africa.With Greek state services nearing collapse, authorities last year decided to round up thousands of undocumented migrants in urban police sweeps and detain them until they can be repatriated.Most of the detention centres are disused former army barracks and many city councils have opposed the plan, arguing that these facilities were never designed to handle security for hundreds of detainees.The migrants themselves have complained of poor conditions inside the centres, particularly a shortage of hot water.A similar protest broke out at a detention centre in the southern city of Corinth last week.A delegation from leftwing party Syriza, the main opposition, which had previously visited the Corinth centre in October reported poor sanitation and medical care in addition to cases of police abuse.