PORTLAND — Portland’s reputation as a bike-friendly city had a flat this week when a cyclist and a motorist got into a fight.

It turns out the motorist who yelled at the cyclist was himself a cycling advocate, trying to give a lecture on bike ethics.

And the cyclist turned out, police say, to be drunk. Reports from the scene say he blew through a red light and then used his bike to assault the driver.

The clash is underscoring the uneasy relationship between folks on two wheels and those on four in a town that regularly wins national recognition for alternate transportation and bike access.

The relationship has suffered, too, after deadly accidents in which motorists turned right at intersections and crushed cyclists trying to go straight through the crossings. Young adults fond of riding kiddie bikes through Portland’s posh West Hills at breakneck speeds have also caused consternation.

This time, there’s a fury of blogging and talk over a confrontation between motorist Colin Yates and bicyclist Steven McAtee. It was first reported by The Oregonian newspaper.

Yates, who describes himself as a cycling advocate who has been involved in the bike industry for more than 30 years, was driving his Subaru through Portland’s Southeast neighborhood Sunday when he saw a bicyclist pass him on the left and then run a stoplight.

The 47-year-old Yates told police he caught up with McAtee, honking his horn and telling him that he shouldn’t be running stoplights because it gives cycling a bad name.

McAtee became irate and challenged Yates to get out of the car, asking him “if he wanted some,” said Sgt. Brian Schmautz, the police spokesman.

McAtee picked up his bike and used it to strike Yates’ car, and when Yates got out of the Subaru, McAtee used his two-wheeler to strike Yates, Schmautz said.

A crowd had gathered, and someone punched McAtee, knocking him to the ground. Said Schmautz: “He goes down like a rock.”

Someone called 9-1-1, reporting a car had hit a bicycle.

Schmautz said a “pretty aggressive crowd” was standing around accusing the motorist of hitting the cyclist.

One witness, who apparently feared the crowd would turn on him, told police he didn’t want to be seen talking to them but later corroborated Yates’ story.

McAtee, a 31-year-old city employee, was charged with assault, criminal mischief, driving under the influence of intoxicants and disorderly conduct.

There has been a sometimes heated discussion in Portland about the responsibilities of motorists and those of cyclists.

It’s a discussion that has been enlivened by the clash between Yates and McAtee, as shown by comments posted on the Internet.

One person wrote that Oregon cyclists “think they own the road and want to be treated like a car, but don’t like following the rules of a car.”

Another encouraged cyclists who feel wronged by motorists to fight back: “I actually think it’s good when a cyclist stands up to a motorist.”

But there were cooler heads as well.

“I get both sides of the picture and I do my best to be courteous whatever mode of transport I’m using that day,” wrote one of them. “It saddens me when I see this aggressiveness on either side when we’re all just trying to get to work/school/home. Be respectful. Be courteous.”