(Adds quotes and description of St Louis rally, details from shooting site)
By Nick Carey and Edward McAllister
ST LOUIS, Aug 24 (Reuters) - Hundreds of people gathered in a St Louis park on Sunday for a rally against police violence that is expected to bring together the family of Michael Brown, killed by an officer in Missouri this month, and the parents of Trayvon Martin, a Florida teenager shot dead in 2012.
The Peace Fest rally in Forest Park takes place one day before the funeral of Brown. The slaying of the 18-year-old black youth by a white police officer, Darren Wilson, in the St Louis suburb 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 reignited the debate about the state of race relations in the United States at a level not seen in recent years, with family and supporters saying it showed the rough treatment that black youths face.
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 on and off 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 on Saturday, 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.
At the St Louis rally, Kevin Harris, 40, a security guard who lives in the area, was sitting with his 11-year-old son Kameron in the crowd on foldout canvas chairs waiting for the Browns to arrive.
Harris wore a T-shirt with the silhouette of a man with his hands up and reading "Please Don't Shoot - R.I.P. Michael Brown" and Kameron was wearing a T-shirt saying "Hands Up Don't Shoot." Both were drinking Gatorade to hydrate in the blazing sun.
"We all need to come together to stop the violence. All the violence, including black on black violence. We are our own worst enemy," Harris said.
In Ferguson, there were few protesters out in the boiling heat on Sunday as temperatures soared to above 100 degrees Fahrenheit (38 C).
The National Weather Service issued an "excessive heat warning" for the St Louis area on Sunday, advising that a "dangerous heat wave" would persist through to Tuesday evening. "Those participating in strenuous outdoor activities will be the most susceptible," the warning read.
Among those who braved the heat were about a dozen African- American women coming from church services and carrying umbrellas to protect them from the sun.
"We just wanted to come here and show our support for the Brown family, even for a little while because of the heat," said Roberta Jackson, 65, a retiree. "This has all been such a shock to the community and it's important to show we care even if we can't stay long."
Meanwhile, supporters of Wilson, the officer who shot Brown, planned a second day of fundraising for him, with a gathering at a St. Louis sports bar where an entirely white crowd 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 on Monday.
Many protesters said they planned to attend the funeral to show solidarity with the Brown family.
"There are going to be thousands of us out there on Monday," said Rita Bonaparte, 40, a nurse who attended the mostly peaceful protest on Saturday. "We're all going to be there to show our support for the Brown family and their need for justice." (Editing by Leslie Adler and Eric Walsh)