"At least four policemen and a schoolchild have been killed in the attack," said Muhammad Faisal, a senior police official in the area.

He said that 18 other policemen are injured.

Local police officials said that the improvised explosive device was hidden in a handcart parked at the roadside near the Lahori Gate area.

"The police van was carrying 20 policemen," said Imtiaz Shah, another police official, adding the vehicle was wrecked in the blast and a group of schoolboys were at the site when the bomb exploded.

"A 12-year-old boy has also been killed in the incident," he said.

Copyright AFP (Agence France-Presse), 2011