Authorities arrested three protesters during a third straight night of unrest in Ferguson as tensions mount amid speculation that a grand jury could decide soon whether to indict the police officer who killed Michael Brown.

St. Louis County police said large groups of protesters blocked traffic three times in the St. Louis suburb Friday night. That happened twice near the police station and once on West Florissant Avenue, the site of violent protests soon after Brown's death. 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.

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Ferguson is on edge as a grand jury weighs whether to indict Ferguson Officer Darren Wilson, who killed Brown, 18, during a violent confrontation on Aug. 9. The white officer's killing of the unarmed black 18-year-old led to protests, some of which turned violent.

Wilson, 28, reportedly told the grand jury that he feared for his life as Brown, who was 6-foot-4 and nearly 300 pounds, 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.

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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.

Eddie Hassaun of the civil rights group Justice Disciples urged protesters not to be confrontational and for police to follow suit.

"We're looking for the action on the other side to be equally as committed to peace in the streets and peace for the demonstrators," Hassaun said.

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.

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.

Concern about the aftermath of the announcement prompted one school district to call off classes for Monday and Tuesday. The Jennings district includes some students who live in Ferguson. It had previously planned to close for Thanksgiving starting Wednesday.

Antonio Henley, owner of Prime Time Beauty and Barber Shop in Ferguson, said concern about the pending announcement is hurting business.

"It's been rough, especially these past few weeks leading up to the decision," Henley said. "Our business has been cut in half because the people in the community are afraid to come around."

The FBI has sent nearly 100 additional agents to Ferguson to help law enforcement agencies, according to a U.S. official who spoke on condition of anonymity because the official was not authorized to discuss the FBI plans.