With Britain's worst riots in a generation spreading to the northwest city of Manchester and three people being killed while defending their community in Birmingham, central England, Cameron said there was a "sickness" in society.
He said London was quieter overnight after 16,000 police flooded the streets and vigilante groups protected stricken neighbourhoods from gangs who have burned down and raided dozens of shops and homes.
"We needed a fightback and a fightback is underway," Cameron told a news conference outside 10 Downing Street after the second meeting of Britain's COBRA security committee in as many days.
"We now have in place contingency plans for water cannon to be available at 24 hours' notice," Cameron said, adding that police had already been authorised to use plastic baton rounds against rioters.
Water cannon have only previously been used in the troubled British province of Northern Ireland to tackle sectarian tensions between the Protestant and Roman Catholic communities.
The violence has raised questions about security ahead of the 2012 London Olympic Games, and it prompted the cancellation of Wednesday's friendly between England and the Netherlands at Wembley Stadium.
Cameron's tough new line comes after he flew back from holiday in Tuscany on Tuesday to take charge of the biggest challenge to the Conservative-led coalition government since it came to power in May last year.
Police have arrested more than 1,100 people across the country for violence, disorder and looting since the riots erupted on Saturday in the north London district of Tottenham after police shot dead a man.
The prime minister dismissed "phoney concerns about human rights" over the issuing by police of photos of some of the suspected rioters. He also called for all those convicted over the disorder to be jailed.
The government has blamed "opportunistic" criminals for the unrest, but the opposition says cuts to social services and the failure to deal with underlying social problems has contributed to the riots.
Some of Britain's most deprived areas erupted late Tuesday, with 200 rioters pelting police with missiles in Toxteth area of the northwest city of Liverpool, which was rocked by huge riots in 1981.
Elsewhere, hooded rioters set fire to buildings in West Bromwich and Wolverhampton in central England and a police station in nearby Nottingham was firebombed. There was also trouble in the western English town of Gloucester.
The focus of Tuesday's violence was Manchester where police were driven back by gangs of hundreds of youths who covered their faces with scarves and ski masks.
Gangs set fire to a girls' fashion store and smashed the glass entrance of the Arndale Centre, Manchester's main shopping mall, allowing hundreds of youths to run off with armfuls of clothes and shoes.
Assistant Chief Constable Garry Shewan of Greater Manchester Police, who joined the force after moving to the city in 1981, called the scenes "senseless violence and senseless criminality on a scale I have never witnessed before."
In Birmingham, Britain's second biggest city, police said they had arrested a man and launched a murder inquiry after an incident in which three Asian men died when they were hit by a speeding car.
Witnesses said the men who died had just come out of a mosque and were protecting their neighbourhood shops after a car was set alight nearby.
"They lost their lives for other people, doing the job of the police," Mohammed Shakiel said outside the hospital where the men were taken, prompting around 200 people to gather in support.
Despite the unrest, police and cricket officials announced that the Test match between England and India, due to take place at the nearby Edgbaston ground, would go ahead as planned on Wednesday.
The only other fatality of the riots so far was a man found with a gunshot wound to the head in a car in the south London suburb of Croydon.
Copyright AFP (Agence France-Presse), 2011