US Attorney General Eric Holder has urged police restraint if there are backlash protests over a decision on proceedings against a white police officer who shot dead a black teenager in Missouri.
The grand jury, which is weighing whether to bring charges against police officer Darren Wilson in the 9 August shooting of Michael Brown, 18, is still in session.
Legal experts say the grand jury can consider charges ranging from first-degree murder to criminal negligence.
Tensions simmered in the St Louis suburb of Ferguson over the case that has become a flash point for US race relations.
Police in riot gear arrested three people in overnight protests that led to scuffles.
St Louis County police said they doused one demonstrator with pepper spay for resisting arrest.
Groups from across the country have said they would take to the streets again in large numbers if charges were not brought, prompting local police to draw up contingency plans for possible fresh violence.
Missouri Governor Jay Nixon has already declared a state of emergency and called in National Guard troops to back up local police in anticipation of protests.
Mr Holder said the Justice Department was providing new guidance to law enforcement authorities about how to approach maintaining public safety while safeguarding the free speech rights of protesters.
He also sent a message to protesters that "the most successful and enduring movements for change are those that adhere to non-aggression and non-violence."
Lawyers for Mr Brown's family say the teenager was trying to surrender when the officer shot him.
Mr Wilson's supporters say he shot Mr Brown in self-defence.
Ron Davis, the top official in a Justice Department office advising Ferguson authorities on crowd response, told Reuters the department has been pressing local police not to use the heavy-handed tactics that spurred anger in August.
Local police used military vehicles and riot gear in August to deal with some protesters in demonstrations that sometimes turned violent.
Hundreds of civil rights lawyers have begun arriving in Ferguson in anticipation of the grand jury decision.
