CINCINNATI - Hillary Clinton on Monday called for an end to the "madness" after the death of three law enforcement officers in Baton Rouge, Louisiana, condemning a series of recent shootings involving police and vowing to hold those who kill police officers legally accountable.

"They represent the rule of law itself. If you take aim at that and at them you take aim at all of us," Clinton told civil rights activists at the annual convention of the NAACP. "There can be no justification, no looking the other way."

The Democratic presidential candidate condemned the killing of three Louisiana law enforcement officers, the latest in a recent string of shootings involving black men in Louisiana and Minnesota and police officers in Dallas.

"We have difficult, painful, essential work ahead of us to repair the bonds between our police and our communities and between and among each other," she said.

A former Marine ambushed police in Baton Rouge on Sunday, killing three law enforcement officers in the attack. Three other officers were wounded, one critically. The shooting was the fourth high-profile deadly encounter involving police over the past two weeks.

Clinton also acknowledged that the violence has gone both ways. "Another hard truth at the heart of this complex matter is that many African-Americans fear the police," she said. "I can hear you."

Clinton has proposed a series of reforms to the criminal justice system, including developing national guidelines on the use of force by police, new investments in bias training, legislation to end racial profiling and funding for body cameras. She has also pushed for cutting mandatory minimum sentences, particularly for drug offences, and providing better support to help the formerly incarcerated find jobs after prison.

Campaigning 250 miles south from where Republicans gathered for the first day of the party's national convention in Cleveland, Clinton poked at Republican candidate Donald Trump's decision not to speak at the NAACP convention.

"My opponent may have a different view but there's nowhere I'd rather be than right here with all of you," she said.

Later Thursday, Clinton faced a series of protests during an address to the annual convention of the American Federal of Teachers, a union that endorsed her over a year ago.

"I share in the urgency and the commitment to actually address these issues," she said, as protesters circled the convention hall shouting, "Hands up, don't shoot."

The black vote was a critical part of President Barack Obama's two national victories, and no state - perhaps other than Florida - demonstrates why better than Ohio, where black voters produce troves of Democratic votes in Cleveland, Columbus and Cincinnati, among a few other smaller cities.

Trump's strategy in Rust Belt states like Ohio is to maximize both white turnout and his share of that vote, while presuming that Clinton simply cannot match Obama's performance in the black community.

"Hillary Clinton's platitudes on gun control and public safety will not ensure an America in which both civilians and law enforcement can feel safe in their own neighbourhoods," said Telly Lovelace, Republican National Committee national director of African-American Initiatives.

Clinton's campaign is launching a major voter mobilization drive during the Republican National Convention, with a goal of getting more than 3 million to register and commit to vote in the 2016 election. In a PBS interview with Charlie Rose that aired late Monday, Clinton argued that Trump was offering voters "simplistic, easy answers" that appeal to people who are anxious and fearful.

"That is, unfortunately, part of our political environment right now," she said. "I understand people who are asking these questions. And I hope by the time we have this election-- it will be clear to voters who they can count on."

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Associated Press writers Ken Thomas in Washington, Bill Barrow in Atlanta and Catherine Lucey in Des Moines contributed to this report.