The head of Bulgaria's largest trade union urged Greek authorities Tuesday to investigate an acid attack that left a Bulgarian immigrant seriously injured.

No one has been arrested for the attack Dec. 22 on Kostadinka Kuneva, a union official with the Athens association of cleaners.

Bulgaria's top union official, Zhelyazko Hristov, wrote an open letter to Greek President Karolos Papoulias, saying Kuneva's colleagues had received death threats because of their union activities.

Kuneva, 44, has been living legally in Greece for seven years, working as a cleaning lady. Police said two men ambushed her on a street outside her central Athens home on her way back from work and splashed acid on her face. She is hospitalized in serious condition.

According to Hristov, 80,000 of the 200,000 Bulgarians living in Greece work without labor contracts and do not have social insurance.

On Monday, about 200 people gathered in Piraeus, the main port near Athens, to protest the attack. Protesters scuffled with police, and four officers were slightly injured. The protest took place outside the company that employed Kuneva, which provides cleaners for Athens trains.