Abdullah, whose campaign has built up momentum on the strength of popular rallies, held a final rally in Kabul on Monday.
In an election stunt, a rarity in the Afghan campaigns, a helicopter circled overhead, dropping hundreds of leaflets with his photo.
"Hey compatriots, wake up, it is time for a big change," was the message written in Dari, Pashtu and Uzbeki, the three most common local languages.
The leaflet also showed Abdullah's election symbol and the serial number as marked on the ballot paper to help voters, a large number of whom are illiterate.
On his part, Karzai, whose power base is in the south, has secured the endorsements of tribal chiefs and the leaders of armed groups who fought in Afghanistan's civil war.
Opinion polls have shown Karzai firmly in the lead with about 45 per cent of the vote, but still short of support to win an outright majority to avoid a runoff against Abdullah, his former foreign minister.
The return overnight from exile of Abdul Rashid Dostum, the former leader of an armed Uzbek group, has added an extra dimension to the election outcome.
Dostum won 10 per cent of the vote during the last election in 2004, and his support could tip the balance for Karzai.
Although it was never clear whether his exile was forced or self-imposed, Karzai's government announced on Sunday that the ex-communist was free to return.
Few of the leader's of Afghanistan's armed groups, which for years fought the Russians and later each other, are viewed with more suspicion by the West than Dostum.
Afghanistan's 17 million voters go to polls on Thursday to elect a president for the second time in history alongside 420 councillors in 34 provinces.
But the Taliban has vowed to disrupt the polls, stepping up its own violent campaign in recent days.
"The election is propaganda from America and its allies ... There is no way the final result of the elections will be acceptable to the Islamic emirate of Afghanistan"
"The election is propaganda from America and its allies," the Taliban said in a statement obtained by Al Jazeera on Monday.
"There is no way the final result of the elections will be acceptable to the Islamic emirate of Afghanistan," it said, refering to the name the Taliban gave to the country during its rule.
A US soldier was killed in the east of the country on Sunday, the Nato-led force in Afghanistan said, the same day three British soldiers were killed in the south.
Also in the south, one person was killed and another eight injured after three mortars were fired into the centre of Kandahar.
Al Jazeera's Zeina Khodr, reporting from Kandahar city, said the authorities had stepped up security in the wake of the attacks.
|Abdullah, centre, has emerged as the
main challenger to Karzai [AFP]
"Provincial authorities have told us that they have sent extra Afghan forces to Arghandab, approximately 10km from Kandahar city, because they believe that those responsible for firing mortar rockets into the heart of Kandahar city yesterday are there," she said.
"We've also seen policemen use American humvees as they patrol the streets [of Kandahar] so there's a heavy security presence in the streets here."
In two of Kandahar's districts there will be no voting in Thursday's elections as the Taliban have too great a control of the area.
Officials have said that insecurity could prevent voting in around seven to nine districts across the country.