Seventeen vehicles were also damaged, some after they were set alight, in the rioting that erupted in Kabul on Monday, the city police chief said, giving the first official toll since the violence started.
Abdul Jamil Junbish said: "We have 12 dead including one police who was badly beaten and died in the emergency hospital."
Mirza Mohammad Yarmand, a director in the interior ministry's criminal investigation department, said that "in total we have arrested 140 people" because "they had violated the rules of demonstrations".
The police paraded for the media some of the arrested people, one of them a 13-year-old, saying many had been caught with stolen property such as television sets.
The violence flared after a US military vehicle slammed into civilian cars, killing at least five people.
Riots, not protests
Witnesses saw US troops open fire, killing civilians and fuelling the protests which quickly turned into riots with large-scale looting and destruction of property.
"I do not consider this a protest - it was a riot, it was a disturbance to disrupt security, to steal, loot, set fire to public property," Junbish said.
Afghans have reacted angrily to
the deaths of the civilians
Junbish said police had not fired at the protesters.
"If they had, the number of casualties would have been tenfold," he said.
Afghanistan's parliament, meanwhile, has demanded the arrest of those responsible for the crash.
In a special session on Tuesday, the lower house decided that "those who are responsible for the traffic accident yesterday should be recognised and handed over to the law", the parliament press officer, Haseeb Noori, said.
Legislators also agreed that the "agitators" of the rioting must be handed over to the law and affected families must be compensated, he said.
The US-led military coalition again expressed its regret about Monday's accident, which it blamed on brake failure.
Aid workers killed
Also on Tuesday, six Afghans working for aid and international organisations were killed in attacks in Afghanistan while two US nationals were slightly wounded, police and the US embassy said.
Hamid Karzai has condemned
the killing of the aid workers
In the first attack, three women working for the ActionAid non-governmental organisation and their driver were shot dead in remote northern Jawzjan province, provincial police chief Juma Khan Hamdard said.
Hamid Karzai, the Afghan president, condemned the murders.
"This is an inhuman act by the enemies of Afghanistan, who kill the innocent people working for the welfare of the Afghan people."
Afghan aid workers
Two Afghans were killed and two US citizens were wounded when a blast hit their vehicle in northern Badakshan province, the US embassy said.
Chris Harris, spokesman for the embassy, said the group was in a two-vehicle convoy carrying staff working for US-based Planning and Development Collaboration International that was working under contract with USAID.
The blast, which appeared to have been caused by a homemade bomb, struck the first vehicle, which was heavily damaged. The second vehicle did not suffer any damage.
There have been several attacks on people working for aid groups or foreign contractors in Afghanistan but they have been rare in the north of the country.
That area has been relatively free of the Taliban-linked violence that has plagued the southern and eastern provinces.