BAGHDAD — An instructor teaching his militant recruits how to make car bombs accidentally set off explosives in his demonstration Monday, killing 21 of them in a huge blast that alerted authorities to the existence of the rural training camp in an orchard north of Baghdad.

Nearly two dozen people were arrested, including wounded insurgents trying to hobble away from the scene.

The fatal goof by the al-Qaida breakaway group that dominates the Sunni insurgency in Iraq happened on the same day that the speaker of the Iraqi parliament, a prominent Sunni whom the militants consider a traitor, escaped unhurt from a roadside bomb attack on his motorcade in the northern city of Mosul.

Nevertheless, the events underscored the determination of the insurgents to rebuild and regain the strength they enjoyed in Iraq at the height of the war until U.S.-backed Sunni tribesmen turned against them. The militants are currently battling for control of mainly Sunni areas of western Iraq in a key test of the Shiite-led government’s ability to maintain security more than two years after the withdrawal of U.S. troops.

While the Iraqi army has been attacking insurgent training camps in the vast desert of western Anbar province near the Syrian border, it is unusual to find such a camp in the center of the country, just 60 miles north of the capital.

The discovery shows that “the terrorist groups have made a strong comeback in Iraq and that the security problems are far from over, and things are heading from bad to worse,” said Hamid al-Mutlaq, a member of the parliament’s security and defense committee.