Pakistani authorities say they have prevented a bomb attack after three vehicles packed with explosives were discovered in the garrison city of Rawalpindi, the home of the country's army.

Rawalpindi and the capital Islamabad were placed on high alert on Friday after the nearly 500kg of explosives were found.

Officials also reported that six people were arrested late on Thursday, including three people suspected of planning to carry out suicide bombings.

"We have recovered three vehicles with a large quantity of explosives from the Dhok Kala Khan area," Rao Iqbal, a senior police official, said.

"We have made some arrests," he added, but did not give any details.

Two security officials, who spoke to the Associated Press news agency on condition of anonymity, said the arrested men were suspected of wanting to target "sensitive installations" and were being questioned by civil and military authorities in Rawalpindi.

The police operation came just days after a car bomb outside the Danish embassy in Islamabad killed at least six people.

On Thursday, a statement posted on a website purportedly used by armed Islamist groups claimed that al-Qaeda had carried out the attack in response to the reprinting of cartoons deemed to be insulting the Prophet Muhammed.