WASHINGTON — The sniper killings of five Dallas police officers brought the nation’s heated presidential campaign to a temporary halt Friday, as a mass shooting prompted Hillary Clinton and Donald Trump to cancel rallies for the second time in recent weeks.

President Obama ordered flags flown at half-staff and denounced the attack as “vicious’’ and “despicable.’’ Obama once again addressed horrific shootings and strains between minorities and police, this time emphasizing the sacrifices police officers make for the rest of society.

“There is no possible justification for these kinds of attacks or any violence against law enforcement,’’ Obama said, just a day after he decried racial disparities in the criminal justice system after police shootings of African-American men in Louisiana and Minnesota.

Each of the political leaders — including Trump, who called for “love and compassion’’ — took pains to strike the right tone at a moment of national anguish.

“We must restore law and order. We must restore the confidence of our people to be safe and secure in their homes and on the street,’’ said Trump, who canceled a speech he had planned to give to Hispanics in Miami.

Trump’s carefully worded response was a striking contrast to his reaction to last month’s Orlando gay nightclub massacre in which 49 people died. Trump drew criticism then for promptly issuing a self-congratulatory tweet “for being right on radical Islamic terrorism.’’

The attack by a black gunman targeting white police officers occurred as marchers in Dallas were protesting separate shooting deaths of two black men by police earlier this week, which appeared similar to shootings that spawned the Black Lives Matter movement.

Trump, in his first public acknowledgment Friday of those shootings, called the deaths of Alton Sterling in Louisiana and Philando Castile in Minnesota “senseless, tragic’’ and said they “remind us how much more needs to be done.’’

Clinton on Friday scratched a planned rally in Scranton, Pa., with Vice President Joe Biden and tweeted that she mourned for the officers.

Clinton delivered a somber speech later Friday night at the African Methodist Episcopal Convention in Philadelphia, at which she read off the names of the officers killed in Dallas as well as the victims of police shootings in recent years. She praised the work of police while calling for criminal justice reform and greater efforts by police and minority communities to listen to each other.

“There is clear evidence that black Americans are more likely to be killed in police incidents than any other group,’’ she said. “We have to do better.’’

In Scranton, a few hours before Clinton’s rally was supposed to start, a smattering of Clinton supporters milled around the dirt and gravel parking lot of a sports complex, absorbing news of the shooting. Some had driven hours to be there and didn’t know it had been canceled.

“It’s not a day for a rally,’’ said Pam Schy, who drove from her home in the suburbs north of Philadelphia.

Speaking from a NATO summit in Warsaw, Obama also hinted at his repeated calls for an assault weapons ban, saying that “when people are armed with powerful weapons, unfortunately it makes it more deadly and more tragic, and in the days ahead we are going to have to consider those realities as well.’’

Some critics, though, accused Obama of playing politics in the aftermath of the Dallas shootings and faulted him for failing to bring about racial unity during his tenure.

Retired neurosurgeon Ben Carson, a Trump surrogate who is African-American, said on Fox Friday morning that there are terrorists and “professional agitators all over our country looking for opportunities.’’

“These opportunities will continue to happen and they will continue to do these things,’’ Carson said. “But I guess the real issue is, you know, the president’s going to start saying, see, gun control.’’

Former House speaker Newt Gingrich also criticized Obama for reacting to the Dallas shootings like “any good liberal college professor’’ with gun control.

At the Capitol, one day after House Speaker Paul Ryan canceled gun control votes because of partisan infighting, Ryan and Democratic leader Nancy Pelosi called for national unity.

“There will be a temptation to let our anger harden our divisions. Let’s not let that happen,’’ Ryan said. “This has been a long week for our country. It’s been a long month for America. We have seen terrible, terrible senseless things.

“Every member of this body — every Republican and every Democrat — wants to see less gun violence. Every member of this body wants a world in which people feel safe regardless of the color of their skin. And that’s not how people are feeling these days.’’

Tracy Jan can be reached at tracy.jan@globe.com.