A few desultory prostitutes tout their wares in the darkening corridor of Kensington Avenue that runs beneath the raised suburban railway known as the "El". The scruffy shops and takeaways that were trading an hour ago are now soundly shuttered. There is little indication of life from the two radios and the computer inside Sergeant Joe Sparks' police car: someone with a half- concealed gun is sighted here, there is word of a robbery there and a suspected drug dealer is seen hovering on a corner somewhere else. Police cars - many with the N-number that signals the narcotic division - circle slowly, like sharks in the gloom. In the office of the 24th police district, the only noise is the clatter of typewriter keys.
Down the side streets, an elderly woman is sweeping the pavement in front of her terraced house. Another is hosing down her steps. Some people have brought out their deckchairs to sit on their verandas in the cool of the evening, and a few children have opened the fire hydrants to cavort in the jets of water.
To Sgt Sparks, this picture of small-scale normality is little short of miraculous. Until 10 days ago, drugs were traded here in broad daylight. "The customers stood in queues, as though they were in a supermarket," says Sgt Sparks. Then the very idea that people could sweep or hose their steps, let alone let children play in the hydrants was unthinkable. They might get caught in the crossfire. The only children on the street then were the smart kids on BMXs wearing $100 (pounds 60) trainers who "work" as look-outs for the dealers.
Then, a patrol could notch up a dozen or more arrests a shift. Tonight, it is possible that Sgt Sparks and his team of 22 officers could end their shift without even one. For the first time in almost two decades, this would count as a success and not failure.
Just two hours before, the new commissioner of the Philadelphia police, Commander John Timoney, had been addressing Sgt Sparks and other members of the East District special operations division in his first roll-call. Until March, Cdr Timoney - a graduate of the old, tough school of policing - had been deputy head of the New York Police Department and one of the leading lights of the celebrated Zero Tolerance campaign there.
Now, he was telling the policemen setting off for night patrol that he wanted "good, honest, aggressive police work" but nothing "overly aggressive, brutal or corrupt". "There's a line in the sand you must not cross." He warned: "If you go in like Attila the Hun, kicking ass all over the place, it won't pay off."
Cdr Timoney has inherited a catalogue of failure. Philadelphia, the fifth largest city in the United States, is one of the few that has not seen a sharp fall in violent crime in the past five years. It has the highest rate of shooting murders in the US (82 per cent of 409 murders last year), one of the highest rates of legal gun ownership, and a drug market that boasts the purest heroin (79.5 per cent).
Crime maps compiled in Mr Timoney's first month showed East District to have the highest concentration of murder, drug dealing and shooting crime in the city. Operation Sunrise is said to be the most sweeping police operation here in the past 20 years and is based on the idea that "drugs are the engine that drives all other crime". Sunrise brings together more than a dozen agencies, local, state and federal, from top-level law enforcement to rubbish collection.
The clean-up began with a bang at 8.30am on Monday morning in the heart of East District with a procession of cars, engines and trucks, lights flashing, to advertise their intentions. It was, says Cdr Timoney, designed partly to protect the police and partly to convey the message to the "95 per cent law-abiding members of the community", which is more than half Hispanic, that they had not been abandoned.
The new toughness from the city authorities and police has spawned some strange alliances. Last week, the mayor, Ed Rendell, said he would consider a proposal from the National Rifle Association (NRA) to make Philadelphia the testbed for their theory that no tougher gun control laws were required, just enforcement of existing laws. Cdr Timoney thinks the NRA as an organisation is "completely nuts", but is all in favour of the mayor's "can-do" interest in enforcement, starting not with the easiest areas, but the toughest in a tough city.
Kensington is typical of a once respectable blue-collar district gone bad, through a succession of factory closures, depopulation, poverty and drugs. Everyone involved in the clean-up say they are in for "the long haul" - 18 months, two years, as long as it takes - and they are going to need that resolve.
"In four to six weeks, perhaps, the dealers and the addicts will get frustrated because we haven't gone away. They'll attack the barricades, assault police cars, target the police. It's going to get messy, but no one's going to back away," says Larry McEllyn, in charge of the Philadelphia division of the Drug Enforcement Administration.
The police have a lot at stake in Kensington, not least because they have a lot to live down. It was they who, in 1985, were responsible for one of the most spectacularly bungled operations anywhere in the US, when they laid siege to two blocks of West Philadelphia occupied by the anarchist Move group, and ended by dropping bombs. "I tell everyone who brings up the Move operation, please stop making that analogy," says Inspector Jerry Daley of Special Operations. "It was a low point for our organisation. Such a huge black eye." But he acknowledges: "It's critically important to ensure that a professional job is done here."
Next month, when the temperature rises in every sense, will be the test.