(Adds rallies on Sunday)
By Nick Carey and Edward McAllister
FERGUSON, Mo., Aug 24 (Reuters) - The parents of Trayvon Martin, the Florida teenager shot dead in 2012, were expected to join the family of Michael Brown, killed by a police officer in Missouri this month, in a rally in St. Louis on Sunday to protest againt police violence.
The demonstration will take place one day before the funeral of Brown, an 18-year-old black youth whose slaying by police officer Darren Wilson in the town of Ferguson on Aug. 9, led to days of unrest and drew global attention to race relations in the United States.
The shooting of Martin by a civilian vigilante was one of the most high-profile incidents of racial tension in the United States in recent years, with family and supporters saying it showed the rough treatment that black youths had to live with.
Representatives of the Brown family said in public flyers that Martin's parents would attend the Sunday demonstration.
In Ferguson, police and demonstrators have clashed nightly for more than a week, drawing criticism of the police for mass arrests and the use of heavy-handed tactics and military gear.
The last four nights have been relatively calm, however, although shortly before midnight, police arrested three people.
The main street of Ferguson was open to traffic and the police presence was down sharply from just 24 hours earlier.
Supporters of the police officer Wilson planned a second day of fundraising for him, with a gathering at a St. Louis sports bar where an entirely white crowd had attended an event for him on Saturday.
The White House said three presidential aides would attend Brown's funeral on Monday.
Speaking on "Face the Nation" on CBS, Democratic Representative Lacy Clay of Missouri, who is due to speak at the funeral, said he had promised Brown's parents he would push for a transparent investigation into his death.
"I'm more concerned that if we do not get to the truth and get to what actually happened and bring justice to this situation, then there's going to be a problem in the streets," he said.
A grand jury began hearing evidence on Wednesday, a process the county prosecutor said could take until mid-October.
The National Guard began a gradual withdrawal from Ferguson on Friday, but authorities remain braced for a possible flare-up of civil disturbances surrounding Brown's funeral.
Many protesters said they planned to attend the funeral to show solidarity with the Brown family.
Rita Bonaparte, 40, a nurse who attended the mostly peaceful protest on Saturday night carrying a sign reading "Don't Shoot - Black Men Are People Too," said she would not "miss the funeral for all the world".
"There are going to be thousands of us out there on Monday," said. "We're all going to be there to show our support for the Brown family and their need for justice." (Editing by Lynne O'Donnell and Angus MacSwan)