Anthony Gray, attorney for the family of Michael Brown, speaks as National Action Network Ferguson chapter president Rev. Carlton Lee, right, listens during a news conference Friday, Nov. 21, 2014, in St. Louis County, Mo. Gray and Lee spoke about preparations as citizens wait for a decision from the grand jury whether to indict Ferguson police officer Darren Wilson in the shooting of Michael Brown.

Image Credit: AP Photo/Jeff Roberson

FERGUSON, Mo. - Authorities arrested three protesters during a third straight night of unrest in the St. Louis suburb of Ferguson as tensions mount amid speculation that a grand jury could decide soon whether to indict the white police officer who killed an unarmed black 18-year-old.

St. Louis County police said large groups of protesters blocked traffic three times in Ferguson on Friday night. That happened twice near the police station and once on West Florissant Avenue, the site of violent protests soon after the Aug. 9 killing of Michael Brown. A motorist lost control and crashed into a light pole during the unrest on West Florissant. Protesters converged on the car, but no one was hurt.

Police spokesman Brian Schellman said three demonstrators were arrested on charges of unlawful assembly after remaining in the street after repeated requests to move.

Ferguson is on edge as a grand jury weighs whether to indict Ferguson Police Officer Darren Wilson, who killed Brown, during a violent confrontation. The killing of Brown led to protests, some of which turned violent, and drew international attention.

Brown's killing reignited a debate over how police treat young black men. It drew attention to racial tensions simmering in Ferguson and other U.S. communities four decades after the civil rights movement of the 1960s. Two-thirds of Ferguson's residents are black but the police force is almost entirely white.

Wilson, 28, reportedly told the grand jury that he feared for his life as Brown, who was 6-foot-4 (1.93-meters) and nearly 300 pounds (136 kilograms), came at him. Some witnesses said Brown was trying to surrender and had his hands up.

St. Louis County Prosecutor Bob McCulloch has said the grand jury decision will be announced this month. His spokesman, Ed Magee, told reporters at around noon on Friday that the grand jury was still in session. Five hours later, though, Magee declined to say whether the panel was still meeting. He did not respond to messages seeking comment Saturday.

The time, date and place for a news conference announcing the decision has not been decided, Magee said.

The size and intensity of protests had dwindled until recently. Arrests have been made three straight nights, mostly for protesters blocking streets.

Calls for peace and restraint emanated from several quarters, including President Barack Obama, Attorney General Eric Holder and civil-rights leaders and business owners.

The most emotional appeal came from Brown's father, Michael Brown Sr.

"Hurting others or destroying property is not the answer," Brown said in the video released by the group STL Forward. "No matter what the grand jury decides, I don't want my son's death to be in vain. I want it to lead to incredible change, positive change, change that makes the St. Louis region better for everyone."

Holder issued a general reminder to police to prepare for demonstrations and to "minimize needless confrontation."

His video message did not explicitly mention Ferguson, but it did reference demonstrations over the past few months that have "sought to bring attention to real and significant underlying issues involving police practices."

Obama also urged Ferguson to keep the protests peaceful, saying all Americans have the right to peacefully assemble to speak against actions they regard as unjust. But, he said, using any event as an excuse for violence is contrary to the rule of law.

The president commented in an interview with ABC News scheduled to air Sunday. The network released his comment about Ferguson on Friday night.

City, county and state leaders on Friday announced a "rules of engagement" agreement between police and roughly 50 protest groups. The pact is aimed at preventing violence on both sides.

Ferguson's police came under criticism for their heavy-handed handling of protests and rioting that broke out after Brown's killing. Police donned riot gear and patrolled in armoured vehicles..

The civil rights organization Advancement Project said more than 70 protest actions are scheduled around the country, including occupying government space in Washington and a gathering at police headquarters in Chicago.

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Salter reported from St. Louis. Associated Press writers Alan Scher Zagier in Clayton, Missouri, and Eric Tucker and Jim Drinkard in Washington contributed to this report.

News from © The Associated Press, 2014