The night time attacks are thought to be the latest strikes by al Qaeda in the world’s biggest oil exporter.
The first assailant tried to drive his cars into the vast Interior Ministry compound in central Riyadh, but it exploded outside one of the gates.
Half an hour later another car bomber tried to enter a centre for special emergency forces but was stopped at the gate before police opened fire and it blew up, the sources said.
A bystander was reportedly killed … a government spokesman said there was no sign of “a large number of casualties” but did not give any more details.
While damage to the Interior Ministry was minor, it was a bold assault on the government body that leads the kingdom’s war against al-Qaeda and other Islamic extremists.
They came just two weeks after al-Qaida leader Osama bin Laden called on his followers to focus attacks on his homeland.
Past militants attacks, including some claimed by al-Qaida, drew heavy criticism when many of the dead were Arab and Muslim.
But Wednesday night’s attack focused on targets associated with Saudi security forces could have been meant to underline militants’ opposition to the government while avoiding killing civilians.
For over a year, Saudi Arabia has been battling a wave of suicide bombings and shootings by the network’s local wing, which is bent on toppling the pro-Western monarchy and driving Westerners out of the birthplace of Islam.
Earlier this month, al Qaeda militants stormed the US consulate in Jeddah, killing five non-US staff in the first attack on a Western mission in Saudi.
The attacks caused oil prices to jump and signaled that Muslim extremists are keeping up their fight despite the kingdom’s crackdown.
Later on Wednesday, Saudi police say they killed seven suspected militants and captured a number of others in a shootout in the Red Sea city of Jeddah.
Police say the gun battle broke out after the militants opened fire with small arms and threw grenades.
Officials say they were suspected of involvement in the evening’s attacks.
The US embassy in Saudi Arabia reiterated its advice to its nationals in the kingdom to exercise extra caution following two explosions that rocked Riyadh.
“American citizens are advised to be aware of their surroundings, exercise caution and monitor news reports closely,” Andrew Mitchelle, deputy spokesman at the embassy in Riyadh, told the AFP news agency.
The blasts sent global oil prices higher, with US crude futures ending up $1.93 at $43.70 a barrel.