By Staff, Agencies

US President Donald Trump and his Democratic challenger Joe Biden traded pre-election barbs Sunday as violent clashes continued to roil the US city of Portland following the fatal shooting of a protestor.

While Trump tried to characterize Biden as weak on crime, his opponent accused him of fanning the flames of violence in a polarized and tense nation.

Saturday's shooting during a pro-Trump rally in the Oregon city followed a week of country-wide protests -- including the cancellation of numerous sporting events -- over the police shooting in Kenosha, Wisconsin of African American Jacob Blake.

The violence in Portland erupted during a rally involving hundreds of vehicles "caravanning throughout downtown Portland," police said. OregonLive reported "clashes" and "tense moments" between demonstrators and counter-protestors.

Photographs from the scene showed the victim wearing a hat with a logo for "Patriot Prayer," described by local media as a far-right group at the center of multiple Portland demonstrations that have ended in violence.

By 10 pm Sunday, about 100 to 150 anti-racism protestors had gathered outside a police building to the east of the city center, waving signs and occasionally throwing projectiles.

Police declared the gathering an illegal assembly and, in a tweet, ordered people to leave the scene, warning of arrests and the use of tear gas.

Videos posted online showed about 20 officers rushing from the building to clear the area, and arresting a handful or protestors.

The Portland clashes followed unrest in Kenosha, where prosecutors accused 17-year-old Kyle Rittenhouse of shooting dead two men and wounding another who were protesting against Blake's shooting.

Trump is due to travel on Tuesday to the Midwestern city to meet law enforcement officials and view damage from unrest triggered by Blake's shooting last weekend.

Wisconsin's governor Tony Evers sent the president a letter asking him to reconsider the visit as it "will only hinder our healing," according to US media reports.