The killing of two woman police officers in Manchester sparked horror across the country today.
Prime Minister David Cameron described it as "a shocking reminder of the debt we owe to those who put themselves in danger to keep us safe and secure".
And the president of the Association of Chief Police Officers, Sir Hugh Orde, said it was "deeply sad news" for the police service.
The unarmed officers were killed while attending a routine incident today, which led to the arrest of one of the country's most wanted men.
Dale Cregan, 29, who was being hunted in connection with the deaths of a father and son in separate attacks, gave himself up to police soon after the incident, in Hattersley.
In a statement released by 10 Downing Street, Mr Cameron said: "The killing of two police officers in Manchester is a shocking reminder of the debt we owe to those who put themselves in danger to keep us safe and secure.
"I want to offer my thoughts, along with those of the whole country, to the families, friends and colleagues at this difficult time."
Home Secretary Theresa May said: "This is a deeply shocking incident and a terrible reminder of the risks that police officers face every day to keep our communities safe.
"My thoughts are with the families of the officers and their colleagues at Greater Manchester Police."
Shadow home secretary Yvette Cooper described the incident as "tragic and deeply distressing news".
Ms Cooper added: "This is a painful demonstration of how police officers put themselves in harm's way every day to protect the public.
"We should pay tribute to the bravery of the two officers today who have given so much in their work to keep us safe, and our thoughts are also with their friends and colleagues in Greater Manchester Police. This is a tragedy that will be felt throughout the country."
In a message on Twitter, Labour leader Ed Miliband said: "My thoughts and condolences are with the families and friends of two police officers killed today while trying to keep their community safe."
Police officers across the country were saddened by the loss of their colleagues.
Sir Hugh said: "The death of two female police officers in Greater Manchester is deeply sad news for the police service. The two unarmed officers were responding to a routine call when faced with a dangerous and deadly situation.
"Whenever police officers and staff lose a colleague that loss is felt right across the police family. The thoughts of everyone in policing are with colleagues in Greater Manchester Police, family and friends of the two officers, at this time."
The chairman of the Police Federation of England and Wales, Paul McKeever, said: "This is devastating news and our thoughts and prayers are with their families, friends and colleagues at this extremely difficult time.
"This tragedy makes us all stop and reflect on the very real dangers police officers face every day serving their local communities.
"These extremely courageous officers have sadly paid the ultimate price for their selfless actions and no words can express our regret, nor comfort to their families. The entire police service throughout the UK mourn their loss.
"I know that my colleagues in Greater Manchester Police Federation will be doing everything they can to offer support and counsel to the relatives and colleagues of these brave officers."
The president of the Police Superintendents' Association of England and Wales, Derek Barnett, said: "Today once again we have seen how police officers pay the ultimate price in losing their lives to protect the public.
"Whilst we still don't know the full circumstances, this tragic situation demonstrates the uniqueness of the office of constable, a duty not to walk away from danger.
"It also serves as a reminder of the risks that officers face on a regular basis.
"At the beginning of their tour of duty these two officers would have had no idea what lay ahead of them.
"I'm sure few words will offer true comfort to the families, colleagues and friends at such a difficult time, but our thoughts and prayers are with them."
Ash Rathband, whose police officer father David was shot and blinded by gun maniac Raoul Moat, called for police to be armed after today's murder.
"It's time for police to be armed in my opinion," wrote Pc Rathband's son. "Yet again another awful incident."
Pc Rathband's twin Darren also called for officers to be armed.
He tweeted: "Give them more than a bloody piece of plastic and some spray. Tragic!"