Pak police have foiled a major terror plot, seizing three explosives-packed vehicles and arresting six people including three would-be suicide bombers near Islamabad, officials said Friday. The discovery of the massive bombs in the army headquarters city of Rawalpindi, which adjoins the capital, comes just days after a deadly suicide car bomb attack outside the Danish embassy in Islamabad. Security forces placed the two cities on red alert following the discovery of the explosives, which amounted to nearly 500 kilograms (1,100 pounds) in total between the three cars. Interior Ministry chief Rehman Malik was quoted by local television stations as saying six "terrorists" including three suicide bombers had been arrested and three explosives-laden vehicles were seized. "It was a major terrorist plot aimed at causing death and destruction in the twin cities," a senior security official told AFP on condition of anonymity. "Security agencies got a tip-off that some explosives-laden vehicles would enter the twin cities and police tightened security on the road and launched checking of vehicles," the security official said. "On Thursday they impounded three vehicles each fitted with 100 to 150 kilograms of explosives... The vehicles were meant to cause terrorist attacks in the twin cities," he said. The official said ony four people had been rounded up. Al-Qaeda claimed in purported Internet statements Wednesday to have carried out the attack on the Danish mission in revenge for the publication in Danish newspapers of cartoons insulting the Prophet Mohammed. The discovery of the bombs will raise fresh concerns over the new Pakistani government's efforts to negotiate with Islamist militants in a tribal region bordering Afghanistan.