Fresh violence has broken out in Afghanistan as the country prepares for elections on Sunday.
Over 1,000 people have been killed in fighting with insurgents this year
Twenty suspected Taleban members were arrested in southern Helmand province when they were caught planting bombs at a dam, the defence ministry says.
Also, seven suspected Taleban rebels and three policemen were killed in two separate incidents overnight.
Over 12 million registered voters will choose from almost 6,000 candidates in the parliamentary and provincial polls.
Security forces have made arrests and seized explosives in several cities across Afghanistan on the eve of polling.
Seven suspected Taleban rebels were killed after ambushing a police patrol in southern Zabul province late on Friday, officials say.
And in Musayi, just outside Kabul, three policemen including a district police chief were killed in an ambush.
The Afghan government has blamed the Taleban and Al Qaeda for the ambush.
Security in the capital has been increased and strict checks are being made on vehicles entering Kabul, the BBC's Bilal Sarwary reports.
A senior security official in the city told the BBC that "we are on a high alert and do not want to take any chances".
Meanwhile, a defence official said that thousands of villagers would have been put at risk if the Kajaki dam, in Helmand province, targeted by insurgents, had burst.
Other cities across the country are less crowded than usual, correspondents say.
The administration of the elections is an additional challenge.
"This is a more challenging election logistically than the presidential elections last year," Sultan Ahmed Baheen, spokesman for the Joint electoral management body, told the BBC.
Ballot boxes have been sent by donkeys, horses and camels
A total of 160,000 polling officials are being deployed for the elections, at 26,000 polling stations across the country.
Ballot papers have been sent by donkeys, horses and camels to far-flung provinces such as Badakhshan, Nuristan, Bamiyan and Kunar.
The Taleban oppose the elections and have vowed to disrupt them.
Commander of US forces in Afghanistan Lt Gen Karl Eikenberry told Reuters news agency that insurgents would not hesitate to attack polling stations.
"We are up against an enemy that will not hesitate to attack unarmed election workers...to try to attack innocent Afghan citizens trying to express their will in a representational government," he said.
But he stressed "tomorrow that election is going to go. There will be some violence, but it's going to go."
A spokesman for the Taleban said, however, that polling stations will not be targeted because of the risk to civilian life, the BBC's Roland Buerk in Kandahar reports.
But in Logar province, officials said that rockets had been found near a polling station, aimed and ready to fire. They were defused.
On Thursday night suspected Taleban militants killed an election candidate in Helmand province.
More than 1,000 people, including seven election candidates, have been killed in fighting with insurgents in the past six months.