Leaders and politicians have been commenting on the shooting of police officers in Dallas city centre - the deadliest attack on US law enforcement since 9/11.
This is how they reacted:
The president called the shooting a vicious, calculated and despicable attack on law enforcement.
He said America is horrified over the shootings and there was no possible justification for them, but added that all Americans should be troubled by frequent police shootings of blacks and Hispanics.
"When people say 'black lives matter', that doesn't mean blue lives don't matter," Obama said, referring to uniformed officers. "That just means all lives matter."
Dallas mayor Mike Rawlings
We have faced challenges in this square, but we must have an attitude of humility. We will face challenges together. pic.twitter.com/mt70W8WtHn
-- Mayor Mike Rawlings (@Mike_Rawlings) July 8, 2016
Cameron's official spokeswoman said: "The Prime Minister is shocked and horrified by the terrible scenes we have seen overnight.
"He is due to be at the Warsaw summit and I think he will have the opportunity to talk to President Obama about it directly.
"I think he would echo what the President has said, which is that these attacks on police officers simply doing their job and trying to keep people safe are horrific and cannot be justified."
House speaker Paul Ryan
The House speaker and Wisconsin Republican urged people to "not lose sight of our common humanity" in an emotional speech.
He said: "An attack on the people who protect us is an attack on all of us. Our hearts are with Dallas Police Department. Our hearts are with the victims, and especially with their loved ones. They wear the badge too.
"I know to be a cop's wife or a cop's husband is to prepare for the worst. But who could've fathomed such horror as this."
I mourn for the officers shot while doing their sacred duty to protect peaceful protesters, for their families & all who serve with them. -H
-- Hillary Clinton (@HillaryClinton) July 8, 2016
Both Trump and Clinton cancelled campaign rallies planned for Friday.
George W Bush
The former president, who lives in Dallas, said he is "heartbroken by the heinous acts of violence in our city last night".
He said: "Murdering the innocent is always evil, never more so than when the lives taken belong to those who protect our families and communities."