PORTLAND, Ore. — About 200 people, some wielding homemade shields, clashed with police early Friday for the third consecutive night as two other Black Lives Matter rallies proceeded peacefully elsewhere in the city, authorities said.

The demonstration with unrest came hours after the city’s Democratic mayor pleaded for protesters to stay off the streets, saying those who barricaded the doors to a police precinct the night before and tried to set it ablaze were not demonstrators, but criminals.

Mayor Ted Wheeler said the violent protesters are also serving as political “props” for President Donald Trump in a divisive election-season where the president is hammering on a law-and-order message. Trump has tried to portray the protesters as “sick and dangerous anarchists” running wild in the city’s streets.

The chaos that started Thursday night and lasted into Friday morning in a residential neighborhood about six miles from downtown marked the 70th night of unrest since May 25, following the George Floyd killing in Minneapolis. Police arrested 12 adults and detained one 17-year-old on suspicion of charges ranging from interfering with a police officer to rioting.

The demonstrations this week are noticeably smaller than the crowds of thousands who turned out nightly for about two weeks in July to protest the presence of U.S. agents sent by the Trump administration to protect a federal courthouse that had become a target of nightly violence.

This week’s clashes nevertheless ratcheted up tensions after an agreement last week between state and federal officials seemed to offer a brief reprieve.