Tony Blair held talks with Hugh Orde at Downing Street on Wednesday morning.
Mr Blair asked for the meeting with the police chief so he could be given the latest security assessment on Northern Ireland.
It follows several months of sporadic sectarian violence at interface areas in east and north Belfast.
Speaking afterwards, Mr Orde said the prime minister's message was "very positive".
"It was extremely constructive. The Prime Minister expressed strong support for the new police service in Northern Ireland."
Mr Blair said the chief constable had a very difficult job but would have "every support we possibly can in meeting the challenge he has".
"I have said to the chief constable he will have our 100% support in making sure we deal with those localised but very serious outbreaks of violence and in the continuing struggle against those people who want to continue with terrorism.
"The vast majority of people don't want anything to do with these localised outbreaks of violence," said the prime minister.
'Assurance'
SDLP leader Mark Durkan met Mr Blair after the chief constable for talks on police reforms.
Speaking afterwards, the deputy first minister said he had been given an assurance that legislation aimed at easing nationalist fears over the new policing arrangements would be introduced in the next session of Parliament.
"We had a very useful meeting with the prime minister and covered quite a bit of ground," said Mr Durkan.
"We have said we want to see these commitments made good, we want to see that legislation tabled in full and he has indicated that will be in the next session."
United States President George Bush's special adviser on Northern Ireland Richard Haass is due in Belfast on Thursday to meet the political parties.
He and Irish Prime Minister Bertie Ahern will hold talks in Dublin with Northern Ireland politicians later this week.
The meeting with the chief constable follows a warning from Police Oversight Commissioner on Tuesday that rising crime and sectarian street violence could wreck a major programme of police reforms in Northern Ireland.
Tom Constantine is overseeing reforms following the changeover from the Royal Ulster Constabulary to the Police Service of Northern Ireland last November.
He said: "Group violence directed at police officers as these try to preserve peace, can threaten the entire concept of police reform.
"This environment of crime and violence creates a foundation for the growing threat of organised crime in Northern Ireland."