MADRID, July 10 (Reuters) - U.S. President Barack Obama has warned that attacks on police over racial bias would hurt the anti-racism Black Lives Matter movement, days after a sniper killed five police officers in Dallas in apparent revenge for police shootings of black people. Obama said although most activists from the Black Lives Matter movement wanted to see better relations between communities and law enforcement, violence and overly broad criticism against police undermined the protest movement. "I want to say to say to everyone concerned about ... racial bias in the criminal justice system that maintaining a truthful, serious and respectful tone is going to help mobilize American society to bring about real change," Obama said. "Whenever those of us who are concerned about failures of the criminal justice system attack police, you are doing a disservice to the cause," he told a news conference in Madrid where he is on a one-day visit.