A car explosion and what appeared to be a suicide attack injured two people, killed the apparent bomber and caused panic among Christmas shoppers in Stockholm.

Stockholm police spokeswoman Petra Sjolander said a car exploded Saturday near Drottninggatan, a busy shopping street in the centre of the city. Shortly afterward, a second explosion was heard higher up on the same street, and a man was found injured on the ground.

He was later pronounced dead.

Sjolander said it was unclear what caused the second explosion and whether the two blasts were linked, but said a police bomb squad has been sent to the site.

Ten minutes before the blasts, Swedish news agency TT received an email saying "the time has come to take action."

According to the news agency, the email referred to Swedish soldiers in Afghanistan and Sweden's silence surrounding artist Lars Vilk's drawing of Muhammad as a dog.

"Now your children, daughters and sisters shall die like our brothers and sisters and children are dying," the news agency quoted the email as saying.

Police said they were aware of the email, which had also been addressed to Sweden's security police, but couldn't immediately confirm a link to the explosions.

Two people were taken to hospital with light injuries. It was not immediately clear in which explosion they were hurt.

Rescue services spokesman Roger Sverndal said the car that exploded contained gas canisters.

Gabriel Gabiro, a former Associated Press staffer, heard the second explosion from inside a watch store across the street and saw smoke coming from the area where the man was lying.

"There was a man lying on the ground with blood coming out in the area of his belly, and with his personal belongings scattered around him," he said.

Gabiro said the blast was "quite loud" and he saw people running from the site.

"It shook the store that I was in," he said. "Then there was smoke and gun powder coming into the store."

"I saw some people crying, perhaps from the shock," he said.

Sweden — which has so far been spared any large terrorist attacks — raised its terror threat alert level from low to elevated in October because of "a shift in activities" among Swedish-based groups that could be plotting attacks there.

The security police said then that the terrorism threat in Sweden remained low compared to that in other European countries, and no attack was imminent.