Finnish police said Lambert assaulted people who tried to break up a fight between the singer and his boyfriend in downtown Helsinki.
Detective Superintendent Petri Juvonen said the fight started at a popular gay bar early Thursday morning, and it spilled into the street, where Lambert was suspected of hitting two people who tried to break it up.
Lambert's boyfriend, "Finnish Big Brother" star Sauli Koskinen, was also detained for questioning. Juvonen said no one was seriously hurt and no weapons were used.
Authorities said the two men were released after a few hours and that no further action would be taken.
The Associated Press contributed to this report.