BAGHDAD, Iraq – A suicide bomber blew himself up in the middle of a crowd of army recruits yesterday, killing 21 other people in the deadliest attack in Baghdad since last week’s election and highlighting a recent shift by insurgents to use human bombs instead of cars.

Insurgents are strapping explosives on the bodies of volunteers to penetrate the network of blast walls, checkpoints, and other security measures designed to block vehicle bombs.

Several such attackers tried to disrupt voting in Baghdad on Election Day but were unable to get into polling stations. On Monday, a suicide bomber walked into a crowd of Iraqi policemen in the northern city of Mosul and detonated explosives, killing 12 of them.

Iraqi authorities initially said the Baghdad recruiting center was attacked by mortar fire, but witnesses reported only a single explosion and the American military said the blast was caused by a suicide bomber on foot.

Attacks have steadily risen since the January 30 elections, when a massive American and Iraqi security operation prevented insurgents from disrupting the vote. Those measures, including a ban on most private vehicles, closing the borders and an extended curfew, were relaxed soon afterward.

An Internet statement posted yesterday in the name of an Al Qaeda affiliate led by Jordanian terror mastermind Abu Musab al-Zarqawi claimed responsibility for the attack on the recruiting center.

“This is the beginning of the escalation we promised you,” the statement said. Its source could not be verified.

Insurgents in recent months have stepped up their offensive against Iraq’s police and security forces, which are less well-trained, armed, and protected than American and other multinational troops, at a time when American military planners are trying to shift more of the security burden onto the Iraqis.

Three Iraqi policemen were killed yesterday in clashes in the western Baghdad neighborhood of Ghazaliya, one of the most dangerous districts of the capital and the scene of numerous gun battles and assassinations over the past six months.

Election workers are still counting ballots for the 275-member National Assembly, 18 provincial councils and a regional Parliament for the Kurdish self-governing region in the north.

No new results were announced yesterday, but a coalition of Kurdish parties is now in second place – raising the possibility that Shiites and Kurds might share power and even open the way for a Kurdish president. Kurdish leader Jalal Talabani already has announced his candidacy for president.

Officials expect a final count by the end of the week.

Partial results released Monday showed the ticket of interim Prime Minister Allawi, a secular Shiite, falling to third place among the 111 candidate lists. A Shiite-dominated ticket endorsed by Iraq’s most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, led with about half the votes, followed by the coalition of Kurdish parties.

If that reflects the final lineup it appears unlikely that Mr. Allawi, a secular Shiite who favors strong ties to America and a tough stand against the insurgents, could emerge as a compromise choice for prime minister when the new assembly convenes by early March.