A bomb exploded on a bustling street of this western city Sunday, killing five people, injuring 29 and deepening concern over security two days after Afghanistan set a new date for its first free elections.

Afghan leader Hamid Karzai (search) blamed enemies of democracy for the blast, which occurred as U.N. and government officials watched a disarmament parade for militia soldiers across town.

Herat police said they had arrested one suspect. But it was unclear who was behind the latest attempt to mar preparations for presidential elections, which authorities pushed back from September to Oct. 9 on Friday.

Police chief Ziauddin Mahmoudi said the time bomb was hidden in a pile of garbage near a building with shops on the ground floor and a police station upstairs.

Officials said the five dead included a 12-year-old boy. One police officer was among 29 others injured.

Dozens of wounded were brought to a city hospital. Four children with bandaged head wounds lay in one ward, two of them wearing oxygen masks.

Six people were in critical condition, said Mohammed Omar Sameem, the head of public health.

There was no immediate claim of responsibility.

Mahmoudi said his officers had detained a man who lived near the scene and found 15 pistols plus ammunition in a search of his home. The suspect, who wasn't identified, denied any involvement in the bombing, Mahmoudi said.

Karzai blamed the attack on "enemies who are desperately trying to derail Afghanistan from the path of reconstruction, peace and democracy."

Violence this year has killed nearly 600 people, including six election workers who died in recent shootings and bombings.

Herat had been spared such attacks, which officials routinely blame on Taliban-led insurgents active mainly in the south and east of Afghanistan.

But the province was shaken in March by a fierce burst of factional fighting that killed 16 people, including a Cabinet minister and a son of powerful Gov. Ismail Khan (search), and prompted Karzai to deploy government troops here.

The United Nations (search) and Karzai have cited repeated clashes among warlord militias across the north and west as proof of the need for disarmament.

The world body irritated Khan, an anti-Soviet resistance hero who helped U.S. forces drive out the Taliban in 2001, by accusing him of stalling the disarmament process. Khan warned that demobilizing his men could leave a security vacuum.

Still, his aides said he supported Sunday's ceremony at a military base on the edge of Herat that marked the start of a drive to dismantle his private army.

Some 750 troops from an armored brigade set for demobilization paraded under the gaze of a deputy defense minister and U.N. officials. Khan stayed away.

Afghanistan's electoral commission cited slow progress in disarmament in its decision Friday to postpone parliamentary elections, originally to be held at the same time as the presidential vote, but now set for April.

The United Nations is concerned that armed factions will use their guns and wealth to fill parliament with their supporters.

The top U.N. official in Afghanistan, Jean Arnault, told reporters in Kabul Sunday that militias might stage attacks so they could argue that they are needed to maintain security.

He urged NATO to speed the deployment of promised extra troops. The 6,500-strong force has been slow to extend its field of operations beyond the capital, although this month it moved into two new northern cities.

Despite his concerns over security, Arnault doubted regional leaders frustrated at the postponement of the parliamentary vote would resort to violence that could disrupt the elections. Some 6.5 million Afghans, out of about 10 million eligible voters, have registered so far.

"There is too much of a national consensus ... for anyone to try to turn it around," Arnault said.