A riot was declared again in Portland after protesters marched to a police union building, blocked a road and started fires.

Officers are working every night to quell unrest in the US state of Oregon’s largest city.

The protest was over almost as soon as it began outside the Portland Police Association building – the same building where protesters were dispersed after a fire was started inside the offices the night before.

On Sunday night, demonstrators marched to the building from a park a few blocks away. The road was blocked with fencing, and flames were seen rising from skips in the middle of the street.

The Portland Police Association is a labour union that represents members of the Portland Police Bureau.

Police forced around 200 protesters away from the building, and live video showed several people being detained. It is unclear how many may have been arrested.

Since George Floyd was killed in Minneapolis, protests have occurred every night in the city for more than 70 days.

Demonstrations this past week have turned violent. Many in the city had hoped for calm after federal agents withdrew more than a week ago.

Three officers were hurt, including two who were taken to hospital, during the efforts to clear the crowd of several hundred people outside the union building late on Saturday. The two injured officers have since been released.

Nine people were arrested during the protest.

Police arrested 24 people during demonstrations overnight on Friday after they said people had defied orders to disperse. Officers said they threw rocks, frozen or hard-boiled eggs and commercial-grade fireworks at the police.

Some demonstrators left obstacles filled with nails in the road, causing extensive damage to a patrol vehicle, police said.

Oregon State Police worked with Portland officers to clear the protesters.

The gatherings this past week had been noticeably smaller than the crowds of thousands who turned out nightly for about two weeks in July to protest over the presence of US agents sent by the Trump administration to protect the federal courthouse.

The Portland Police Association building is located about five miles north of the federal courthouse.