The fatal shootings of five Dallas police officers touched off an outpouring of grief across Massachusetts Friday, prompting calls for unity from state leaders and putting law enforcement on heightened alert.

Governor Charlie Baker decried the attacks as a “senseless and heinous crime’’ and ordered flags on all state buildings lowered to half-staff. On Beacon Hill, he made an emotional plea on behalf of the families of the five officers and the victims of police shootings that spurred the Dallas protest.

“The people, the innocent people, who died this week, in Minnesota, in Louisiana, and Texas, were somebody’s son, daughter, father, friend, sibling,’’ Baker said. “For all the yelling and the screaming, and all the blame that’s going to get thrown around in the next 24 to 48 hours, to the people who lost somebody who mattered to them, that all is going to sound pretty cheap.’’

In Boston, police required two-officer patrol units, a precaution to remain in place through at least Friday. The Boston Regional Intelligence Center warned officers to remain “extremely vigilant’’ on and off duty, citing social media posts by known gang members that called for people to target officers. At least one post suggested that individuals follow officers home from the stations, though it was not clear whether the threats were credible.

Mayor Martin J. Walsh said Boston police were on high alert after the deadliest attack on US law enforcement since Sept. 11, 2001.

“I think the police are going to be watching what’s going on,’’ he said. “This was a terrorist attack, and in a terrorist attack, you’re always worried about the copycat scenario.’’

Walsh lamented that the country was “getting more and more divided’’ and called on elected officials to help bridge that gap. He called the office of Dallas Mayor Mike Rawlings, a Boston College graduate, to offer his support for the police department.

“This week has been a very tragic, sad week for this country,’’ he said.

On Friday night, Walsh and Attorney General Maura Healey joined up with the Twelfth Baptist Church Peacewalkers for their weekly walk through Roxbury, along with Boston police officers, to promote community building between residents and law enforcement.

“There’s way too much of an us vs. them mentality,’’ Healey said.

The group of about 35 people from the church, the mayor’s office, and the Police Department walked around for more than an hour, as Walsh and Healey posed for pictures, shook hands, and teamed up for a game of two-on-two basketball with local youths.

Roxbury resident Cheyenne Hutton, 50, said it was an important day for Walsh to be in the community.

“He’s visible; he’s out here,’’ Hutton said.

Another peaceful gathering took place in the South End where members of the local black community gathered for a vigil to mourn the black victims fatally shot by police this week and to discuss ways to move forward.

Daunasia Yancey, founder and lead organizer of Black Lives Matter Boston, said the vigil was an opportunity “for black people come together to grieve, to mourn, to process the violence that we are seeing daily now from the state, from police officers, against black people.’’

The crowd of about 200 people met at the Union United Methodist Church.

More than a dozen police officers, some wearing bulletproof vests, stood outside or sat in cars and followed the crowd as it relocated. But the evening remained peaceful.

One of the five slain officers worked for the Dallas Area Regional Transit, known as DART. The head of its counterpart agency in Massachusetts, MBTA Transit Police Superintendent Richard Sullivan, extended his condolences to the officer, Brent Thompson, 43, and his loved ones.

Sullivan said the violence provides an opportunity to improve relations between police and the public.

“With all my heart, I believe that members of the community can support their local police, but at the same time advocate for rooting out biases that may exist among certain law enforcement people,’’ Sullivan said. “Now is not the time for divisiveness.’’

In many communities, police stepped up patrols after the attacks.

“Officers must constantly remain aware of their surroundings to ensure their personal safety,’’ said Cambridge City Manager Richard Rossi, who called the city’s police force a “model department for respecting the rights of all citizens and ensuring the public’s safety during times of protest and other difficult incidents.’’

North of Boston in Lawrence, Mayor Daniel Rivera said police are in a “heightened state’’ of vigilance, and that officers are being urged to be “more alert and cautious’’ on patrol this weekend.

In Watertown, where the shootings stirred memories of the police shootout after the Boston Marathon bombings, officers will be providing back up to each other on routine calls, Police Chief Michael Lawn said.

“They’ll be keeping an eye on each other,’’ Lawn said. “We’re telling our officers to be vigilant and aware of their surroundings.’’

Lawn said some residents had dropped by the station, or sent e-mails, to show their support.

“It goes a long way for the officers who are out there working to know they are supported,’’ he said.

In Lowell, police held a moment of silence for the victims, pulling their cruisers to the road side and flashing their emergency lights. In Springfield, police, clergy, and city officials joined together in prayer on the City Hall steps.

“In the spirit of unity and tolerance, we stand against hate,’’ Mayor Domenic Sarno said.

The state’s congressional delegation reacted quickly to the shootings, offering condolences and renewing calls for tightening gun laws.

“This whole week has been heartbreak,’’ US Representative James McGovern said. “I think the question in the aftermath of this is whether or not again we do nothing more than a moment of silence, or whether or not we actually look into some concrete things to do.’’

John R. Ellement and Evan Allen of the Globe staff and Globe correspondents Meg Bernhard and Dylan McGuinness contributed to this report.