OSLO, Norway — A bomb ripped open buildings in the heart of Norway's government Friday, and a man dressed as a police officer opened fire at an island youth camp connected to the ruling party. At least seven people were killed in the blast and nine more in the camp shootings, the peaceful nation's worst violence since World War II.

Oslo police said 9 or 10 people were killed at the camp on Utoya island, where the youth wing of the Labor Party was holding a summer camp for hundreds of youths. Acting Police Chief Sveinung Sponheim says a man was arrested in the shooting, and the suspect had been observed in Oslo before the explosion there.

Sponheim said police were still trying to get an overview of the camp shooting and could not say whether there was more than one shooter.

Aerial images broadcast by Norway's TV2 showed members of a SWAT team dressed in black arriving at the island in boats and running up the dock. Behind them, people stripped down to their underwear swam away from the island toward shore, some using flotation devices.

In Oslo, the capital and the city where the Nobel Peace Prize is awarded, the bombing left a square covered in twisted metal, shattered glass and documents expelled from surrounding buildings.

Most of the windows in the 20-floor high-rise where Prime Minister Jens Stoltenberg and his administration work were shattered. Other buildings damaged house government offices and the headquarters of some of Norway's leading newspapers.

Stoltenberg was working at home Friday and was unharmed, according to senior adviser Oivind Ostang.

Sponheim wouldn't give any details about the shooting suspect, who he said was dressed in a police uniform when he opened fire into a crowd of youths.

A spokesman for Stoltenberg's Labor Party, Per Gunnar Dahl, said he couldn't confirm that there were fatalities at Utoya, about 60 miles (100 kilometers) northwest of Oslo. The party's youth wing organizes an annual summer camp on the island, and Stoltenberg had been scheduled to speak there Saturday.

"There are at least five people who have been seriously wounded and have been transported to a local hospital," Dahl said. He said the shooting "created a panic situation where people started to swim from the island" to escape.

Police blocked off roads leading to the lake around Utoya. An AP reporter was turned away by police about 5-6 kilometers from the lake, as eight ambulances with sirens blaring entered the area.

In Oslo, police said the explosion was caused by "one or more" bombs, but declined to speculate on who was behind the attack. They later sealed off the nearby offices of broadcaster TV 2 after discovering a suspicious package.

Ian Dutton, who was in a nearby hotel, said the building "shook as if it had been struck by lightning or an earthquake." He looked outside and saw "a wall of debris and smoke."

Dutton, who is from New York, said the scene reminded him of Sept. 11 — people "just covered in rubble" walking through "a fog of debris."

"It wasn't any sort of a panic," he said, "It was really just people in disbelief and shock, especially in a such as safe and open country as Norway, you don't even think something like that is possible."

Public broadcaster NRK showed video of a blackened car lying on its side amid the debris. An AP reporter who was in the office of Norwegian news agency NTB said the building shook from the blast and all employees were evacuated. Down in the street, he saw one person with a bleeding leg being led away from the area.

The explosion occurred at 3:30 p.m. (1330 GMT), as Ole Tommy Pedersen stood at a bus stop 100 meters (yards) away.

"I saw three or four injured people being carried out of the building a few minutes later," Pedersen told AP.

The United States, European Union, NATO and the U.K., all quickly condemned the bombing, which Britain's Foreign Secretary William Hague called "horrific" and NATO Secretary-General Anders Fogh Rasmussen deemed a "heinous act."

In Washington, State Department spokeswoman Heide Bronke Fulton called the violence "despicable." There has been no confirmation of any U.S. casualties, she said. The U.S. Embassy in Norway warned Americans to avoid downtown Oslo.

The U.S. has offered help to Norwegian authorities but there has been no specific request for assistance, she said.

The attacks come as Norway grapples with a homegrown terror plot linked to al-Qaida. Two suspects are in jail awaiting charges.

Last week, a Norwegian prosecutor filed terror charges against an Iraqi-born cleric for threatening Norwegian politicians with death if he is deported from the Scandinavian country. The indictment centered on statements that Mullah Krekar — the founder of the Kurdish Islamist group Ansar al-Islam — made to various news media, including American network NBC.

Terrorism has also been a concern in neighboring Denmark since an uproar over cartoons of the Prophet Muhammad six years ago. Danish authorities say they have foiled several terror plots linked to the 2005 newspaper cartoons that triggered protests in Muslim countries. Last month, a Danish appeals court on Wednesday sentenced a Somali man to 10 years in prison for breaking into the home of the cartoonist.

Associated Press reporters Louise Nordstrom in Stockholm, Matthew Lee and Rita Foley in Washington and Bjoern H. Amland in Hoenefoss, Norway, contributed to this report.