BAGHDAD -- 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 underscoring 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 US military said the blast was caused by a suicide bomber on foot.
Attacks have steadily risen since the Jan. 30 elections, when a massive US 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 not as well trained, well armed, and well protected as American and other multinational troops, at a time that US military planners are trying to shift more of the security burden to 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 Shi'ites and Kurds might share power and perhaps open the way for a Kurdish president. Kurdish leader Jalal Talabani 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 Iyad Allawi, a secular Shi'ite, falling to third place among the 111 candidate lists. A Shi'ite-dominated ticket endorsed by Iraq's most influential Shi'ite 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 Allawi, who favors strong ties to the United States and a tough stand against the insurgents, could emerge as a compromise choice for prime minister when the new assembly convenes by early March.
Shi'ites are thought to constitute about 60 percent of Iraq's 26 million people, and the Sistani-backed ticket has signaled it wants the premiership.
Many Sunni Arabs are thought to have stayed home on election day, out of fear of insurgent attack or heeding calls from Sunni clergy to boycott the polls.
Sunni Arabs, about 20 percent of the population but the dominant political force for decades, form the core of the insurgency.
Yesterday, the major Sunni party condemned violence last month at a US detention center in southern Iraq in which American guards fired on prisoners to quell a riot. Four prisoners were killed, the US command said. The Iraqi Islamic Party demanded that those responsible face criminal charges and warned the US military against such actions. The US military already announced its own investigation into the decision to use lethal force during the Jan. 31 riot.
In other violence, gunmen fired on a car carrying an Iraqi politician who gained notoriety last year when he attended a conference on terrorism in Israel. Mithal al-Alusi, head of the small Nation party, escaped injury but two of his sons were killed, police said. Also yesterday, a militant group said in an Internet statement that it had executed a female Italian journalist abducted in Iraq for spying on ''holy fighters."
There was no way to verify the authenticity of the assertions, which offered no proof that Giuliana Sgrena, a 56-year-old reporter, was dead or had been held by the group. She had been kidnapped Friday in Baghdad.