Protesters in Portland gathered outside Democrat mayor Ted Wheeler's apartment Monday night to 'celebrate' his birthday by letting off fireworks and singing 'happy tear gas to you'.

A fire was lit in the street outside the 58-year-old's apartment block in an upmarket area in the north of the city, as around 200 people gathered while blasting music, sounding horns, banging pots and pans, and playing drums.

Activists - some of whom were carrying balloons and wearing party hats - danced to the Macarena and were heard shouting slogans including 'Black Lives Matter'.

Police arrived around 11pm after fires were started in the street, declared the scene a riot, and ordered the demonstrators to leave, KOIN reported.

When they refused, officers charged at the crowd and made arrests. It is unclear exactly how many people were arrested.

Videos showed a trash can and picnic table were set on fire, with other pieces of furniture from outside nearby businesses added to the flames. Marchers stood nearby holding a large sign that said 'resign'.

Additional footage showed windows had been smashed at several nearby businesses, including what appeared to be a dentist's office.

The building had also been daubed with graffiti while a small fire had been started in the lobby.

Activists also danced to the 'Macarena' and sang 'Happy tear gas, war crimes Wheeler/Happy tear gas to you,' to the tune of 'Happy Birthday', KATU reported.

Monday marked the 95th consecutive day of demonstrations in Portland, which began after the killing of George Floyd during his arrest in Minneapolis.