The two groups merged around 9 p.m., with marchers moving traffic barricades into a roadway as authorities urged caution to anyone driving in the city's downtown area.
"Officers have provided multiple additional public safety warnings to the group," police tweeted. "Individuals have continued to put items in roadway, including nails."
Police said they had made several arrests, including one involving someone who damaged a parking meter with a hammer.
Of the eight arrested, police said one was a driver who drove over a barricade and into a police bike lane. No injuries were reported in that incident.
Throughout the night, police kept giving public safety warnings for demonstrators to keep moving.
"Keep moving up the hill," police could be heard saying in one video.
By 10:20 p.m., Seattle police said at least eight people had been arrested. The arrests were for pedestrian interference, obstruction, assault on an officer, reckless driving and criminal mischief, police said.
“We are not scared of whoever gets elected. It’s not going to stop us,” one protester who declined to give their name told the Seattle Times.
The Office of Police Accountability in Seattle encouraged the Seattle Police Department “to allow demonstrators to freely exercise their First Amendment rights” and advised demonstrators to protest peacefully.
Earlier in the day, Seattle Mayor Jenny Durkan said she knew people had questions about what could happen in Seattle on Election Day and in the following days, Q13FOX reported.
“In one of the most challenging years in our city’s history, we have a president that has consistently incited hate, fear, and violence," Durkan said. "This is why my office has been closely coordinating with the Governor, County Executive, and City departments for a safe and secure Election Day and planning for the days that follow.”
Durkan said law enforcement agencies do not have any intelligence to indicate there are any specific threats for the city.
Large protests were also reported in Portland, Ore., where hundreds marched through the city on Tuesday chanting among other things, “This is what democracy looks like.”
Organizers told FOX12 the demonstration would be peaceful and that regardless of the presidential election result, they would continue demonstrating in support of racial justice.
Hundreds of businesses in cities across the U.S. boarded up their doors and windows ahead of the election, fearing the vote could lead to the sort of violence that broke out after Floyd's death.
The Associated Press contributed to this report.