* Karzai calls on voters to turn out in their millions
* Gunmen battle police in morning Kabul bank siege
* Taliban threaten to shut roads
KABUL, Aug 19 (Reuters) - Taliban fighters will not wreck Afghanistan's presidential election, President Hamid Karzai said hours before polls open, after fighters clashed with police in the centre of Kabul and threatened to shut the country's roads.
"I hope that tomorrow our countrymen, millions of them will come and vote for country's stability, for the country's peace, for the country's progress," Karzai said late on Wednesday after a small ceremony for the country's Independence Day holiday.
"Enemies will do their best, but it won't help."
Earlier on Wednesday, gunmen stormed a bank building in central Kabul and battled police for hours in what the Taliban said was one of many attacks it had planned for the capital.
The brazen early morning raid was the third major attack in Kabul in five days, shattering the calm in a city which had been secure for months but is now tense and dotted with checkpoints.
Polls show Karzai leading but likely to fall short of the outright majority needed to avoid an October run-off, after his main challenger, ex-foreign minister Abdullah Abdullah, ran a stronger than expected campaign.
Violence could raise the chance of a run-off by suppressing turnout in southern areas where Karzai draws his support -- or even jeopardise the legitimacy of the poll altogether.
In southern Kandahar province -- Karzai's home and the birthplace of the Taliban -- two election workers were killed in a bomb blast, an election official said.
Fearing more election-related violence, officials in Kandahar city said they would close roads to normal traffic for Thursday's poll, allowing only election workers and observers, vehicles transporting voters, and the media to travel freely.
The United Nations asked the government to lift a decree ordering a blackout of foreign and domestic media coverage of any violence during Thursday's polls. The government says it ordered the blackout so Afghans won't be frightened away.
SUICIDE BOMB VESTS
In a statement on a Taliban website (www.alemara.org) the Islamist group said 20 suicide bombers had infiltrated the capital, preparing attacks to thwart the election. Another statement said the militants were closing roads countrywide.
"The mujahideen will bear no responsibility for whoever gets hurt," it said.
Taliban spokesman Zabihullah Mujahid said five gunmen, some wearing bomb vests, carried out Wednesday's Kabul raid, which follows a pattern of similar recent strikes in southern towns.
Police said three fighters were involved. Security forces took reporters into a nearby compound and showed them the bullet-riddled bodies of three fighters killed in the clash.
The raid came a day after a suicide car bomber killed eight people in the capital, the second such strike in four days. Such attacks had been common in the south but had not taken place in comparatively secure Kabul for months.
Many Afghans insist they will vote in spite of the threats.
"Why should I be afraid? This is Kabul, in Kabul there is security," said Noor Agha, 30, near the scene of Wednesday's siege. "I will vote tomorrow, I'm not afraid."
In the runup to the poll, Karzai has alarmed Western donors by lining up endorsements from former militia chieftains whose armed factions once held sway. One of the most controversial, Uzbek General Abdul Rashid Dostum, returned from exile on Sunday night and held a giant rally on behalf of Karzai on Monday.
Dostum won 10 percent of the vote in the last election, and his support could prove key if Karzai secures a first round win. The United States and United Nations both expressed concern at his return and the prospect of him joining a future government.
In an interview at his palace in the north of the country, Dostum told Reuters he had made no personal agreement to support Karzai but had come back to persuade his supporters to vote.
Abdullah has campaigned energetically, with well-attended rallies across the country, and has been polling about 25 percent to about 45 percent for Karzai. Nevertheless, he draws most of his support from the north and would face an uphill struggle to get enough southern support to win the presidency.
The election is a test for U.S. President Barack Obama's tactic of sending thousands of extra troops to Afghanistan to reverse Taliban advances. More than 30,000 extra U.S. troops arrived this year, raising the total foreign force above 100,000 for the first time, including 63,000 Americans.
"The election is difficult to hold in a war-time country," Richard Holbrooke, Obama's envoy for Pakistan and Afghanistan, said during a visit to Pakistan.
"We're not sure how many polling stations will be closed because of security. Taliban has said they're going to try to close them all. But I don't know how many they will succeed in closing," he said.
"No election is perfect. Don't expect a perfect election."
(Additional reporting by Jonathon Burch, Sayed Salahuddin, Ahmad Masood and David Fox in KABUL; Golnar Motevalli in SHIBERGHAN; Ismail Sameem in KANDAHAR and Adam Entous in KARACHI; Writing by Peter Graff; Editing by Paul Tait)
An Afghan national army soldier keeps watches at the vehicle check point in Kandahar province, August 19, 2009. With Afghans to vote on Thursday in the country's second presidential elections since ...
