By Nick Carey and Edward McAllister FERGUSON Mo. (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 against police violence. The demonstration will take place one day before the funeral of Brown, an 18-year-old black youth whose slaying by a white 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 live with. Representatives of the Brown family said in public flyers that Martin's parents would attend the Sunday demonstration.