KABUL, Afghanistan – The deadliest insurgent attack since the U.S.-led invasion of Afghanistan in 2001, destroyed a bus full of police instructors at Kabul’s busiest transportation hub on Sunday, killing 35 people and wounding 52, officials said.

The enormous suicide blast, which raised the specter of an increase in Iraq-style bombings with heavy casualties, was at least the fourth attack against a bus carrying Afghan police or army soldiers in Kabul in the last year. The blast sheared off the bus’ metal sidings and roof, leaving a charred frame.

“Never in my life have I heard such a sound,” said Ali Jawad, a 48-year-old who was selling phone cards nearby. “A big fireball followed. I saw blood and a decapitated man thrown out of the bus.”

The explosion was the fifth suicide attack in Afghanistan in three days, part of a sharp spike in violence around the country. In the south, in Kandahar province, a roadside bomb killed three members of the U.S.-led coalition and an Afghan interpreter. The soldiers’ nationalities were not released, but most in the coalition are American.

Condemning the Kabul attack, President Hamid Karzai said the “enemies of Afghanistan” were trying to stop the development of Afghan security forces, a key component in the U.S.-NATO strategy of handing over security responsibilities to the Afghan government.

A self-described Taliban spokesman, Qari Yousef Ahmadi, said a Taliban suicide bomber named Mullah Asim Abdul Rahman caused the blast. Ahmadi called an Associated Press reporter from an undisclosed location. His claim could not be verified.

Zemeri Bashary, the spokesman for the Interior Ministry, said late Sunday that 35 were killed and 52 wounded in the blast. Karzai’s office said 22 police instructors died, indicating that 13 of the dead were civilians.

At least one person survived the 8:10 a.m. bus blast. Nasir Ahmad, 22, a janitor at the police training academy, was sitting in the back of the bus when the bomb exploded.

Associated Press reporters Noor Khan in Kandahar and Amir Shah in Kabul contributed to this report