State television showed footage on Thursday of the truck, seized after a clash on Tuesday in which two wanted fighters were killed.
Officials say the raid by security forces thwarted an imminent attack during the Muslim holiday of Eid al-Fitr, less than three weeks after suspected al-Qaida bombers killed at least 18 people in a residential compound in the Saudi capital.
An interior ministry source said the truck had 1,267 kg of ammonium nitrate-based explosives in the back when it was seized. A further 1,377 kg of explosives was found nearby, along with more than a thousand rounds of ammunition.
Television footage showed a pick-up truck, painted dark brown and marked with military insignia. Authorities also showed what they said were forged emblems of state security agencies, and military uniforms.
Witnesses have said the attackers who struck on 9 November at al-Muhaya compound on Riyadh's eastern desert outskirts got through a Saudi National Guard checkpoint because their vehicle had military markings.
The attackers used military
markings to get through security
The two men killed on Tuesday were Abd al-Mohsin Abd al-Aziz al-Shabanat and Musaid Mohammed Daidan al-Subai, the interior ministry said.
Shabanat was shot and killed as he fired at security forces from the boot of a car while Subai blew himself up with a hand grenade, it said. Shabanat's corpse and Subai's mangled remains were also shown on television.
Security forces were still hunting down an unspecified number of fighters who escaped, the ministry said.
Saudi Arabia has cracked down heavily on suspected fighters since May, when triple suicide bombings in Riyadh killed 35 people. It says Saudi-born Usama bin Ladin's al-Qaida network is trying to topple the royal family which rules the world's biggest oil exporter.
Newspapers hailed Tuesday's clash as the fourth success by security forces this year in thwarting planned attacks.
Authorities have announced huge arms seizures in recent months across the kingdom, including the holy city of Makka, where Interior Minister Prince Nayif bin Abd al-Aziz said fighters were planning attacks against Muslim pilgrims.
But much of the country remains on guard.
In the capital, extra concrete blocks were put round the Faisaliah tower - one of two landmark skyscrapers - over the three-day Eid holiday. Some hotels tightened security and expatriate compounds kept up high defences against attack.