A man who shot a police officer in his patrol car has pledged his allegiance to Islamic State, police say.
Police Captain James Clark said the alleged shooter, Edward Archer, "pledges his allegiance to Islamic State, he follows Allah and that was the reason he was called on to do this".
Officer Jesse Hartnett was seriously wounded after being shot three times in the arm in the apparently unprovoked ambush.
It was described by police as an "attempted assassination".
Officer Hartnett shouted "I'm bleeding heavily" into his radio before managing to chase and wound the suspect with a shot to the buttock. Archer's condition is unknown.
The arrested man, who lives in Philadelphia and has a criminal record, is said to have fired a total of 11 shots that hit the officer and his car, police say.
"He just came out of nowhere and started firing on him," Police Commissioner Richard Ross said. "He just started firing with one aim and one aim only, to kill him."
CCTV stills show the gunman, dressed in a white robe, walking up to the driver's side door. He was firing all that time, police said.
Commissioner Ross said Archer carried out the attack "in the name of Islam" but there was no evidence that he had worked with anyone else.
Officer Hartnett, 33, was taken to Penn Presbyterian Hospital and, despite being in a stable condition, will require several surgeries.
"I don't know how this officer survived," Commissioner Ross said. "This is absolutely one of the scariest things I've ever seen."
The shooter used a gun that had been stolen from a Philadelphia police officer's home several years ago - but not by the suspect.
"We know it was stolen, how many hands it may have passed through in the last couple of years, we do not know," Commissioner Ross said.
Archer's mother, Valerie Holliday, told The Philadelphia Inquirer her son has been hearing voices recently and that the family asked him to get help. She described him as devout Muslim.
However, the Council on American-Islamic Relations said Archer did not appear to be an observant Muslim.
Philadelphia Mayor Jim Kenney told reporters he did not believe Archer's actions reflected Islamic thinking.
"In no way shape or form does anyone in this room believe that what was done represents Islam," he said. "This was done by a criminal with a stolen gun".
The mayor added: "There are just too many guns on the streets, and I think our national government needs to do something about that."
The shooting came just days after President Barack Obama announced a series of executive actions aimed at tightening US gun laws.
