Vigilante groups have threatened to "retake" the independent zone in Seattle where protesters have set up barricades to keep police out.
An event promoted on Facebook suggests more than 500 bikers plan to attack the so-called Capitol Hill Autonomous Zone, which they say is "illegally occupying public property and terrorising small businesses".
The zone, established a week ago, covers a six-block area surrounding the abandoned East Precinct police station. Some protesters are camping overnight and first aid stations, food stalls and sound systems have been set up.
The Facebook event, hosted by Prntly, a pro-Trump site, and scheduled for July 4, refers to protesters as "communist kids" and suggests that non-bikers with other vehicles could also join in.
Tweets and articles posted online have threatened the involvement of the Hell's Angels and Mongols motorcycle gangs, though it is not clear how credible these are.
Across the US, cars and lorries have been used as weapons during the wave of nationwide protests. Cars have been driven into crowds during two protests in Seattle, with some injuries reported.
The zone has become a target for pro-Trump social media users, who have called the protesters "terrorists" and claim they represent "Antifa", the loose affiliation of left-wing groups that has been blamed for the protests spreading across the US.
Right-wing vigilantes have posted pictures and videos online claiming to have "infiltrated" the zone, which is blocked off to traffic but is not performing checks on visitors.
While Mr Trump has threatened to "take back" the zone, Seattle's politicians have been more conciliatory.
Jenny Durkan, the Democratic mayor who visited the area last week, told the president "go back to your bunker" and said sending in the army would be unconstitutional.