PORTLAND, Oregon, USA — A fatal shooting after clashes in Portland, Oregon, came on the heels of the shooting deaths of two people during confrontations in Kenosha, Wisconsin, earlier in the week.

The fatal shooting in Portland over the weekend led President Trump to unleash a torrent of tweets and attacks on Sunday, capping a volatile week of street violence that is becoming a major theme in the final weeks of the 2020 campaign.

On Saturday, a man affiliated with a right-wing group was shot and killed as a large caravan of supporters of Mr. Trump drove through downtown Portland, where nightly protests have unfolded for three consecutive months. No suspect has been publicly identified and the victim’s name has not been released.

The shooting came in the same week that a 17-year-old armed with a military-style weapon was charged with homicide in connection with shootings during a protest in Kenosha, Wisconsin, that left two people dead and one injured.

The pro-Trump rally in Portland drew hundreds of trucks filled with supporters and adorned with Trump flags into the city. At times, Trump supporters and counter-protesters clashed in the streets, with fistfights occurring and Trump supporters shooting paintball guns from the beds of pickup trucks as protesters threw objects at them.

Mr. Trump on Sunday morning posted or reposted a barrage of tweets about the clashes in Portland, with many of them assailing the city’s Democratic mayor, Ted Wheeler. The president retweeted a video showing his supporters shooting paintballs and using pepper spray on crowds in Portland before the fatal shooting. Mr. Trump wrote that “the big backlash going on in Portland cannot be unexpected,” a remarkable instance of a president seeming to support confrontation rather than calming a volatile situation.

Joe Biden issued a statement on Sunday accusing Mr. Trump of “recklessly encouraging violence,” while condemning “violence unequivocally” himself. “I condemn violence of every kind by anyone, whether on the left or the right,” Mr. Biden said. “And I challenge Donald Trump to do the same.”