Abdullah, whose campaign has built up momentum on the strength of popular rallies, held a final rally in Kabul on Monday.
More than 10,000 people poured into the National Olympic Stadium wearing blue baseball hats, waving blue flags, carrying pictures of Abdullah and chanting his name.
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 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.
Bays said Abdullah was "the clear main challenger to Hamid Karzai".
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.
His office said eight candidates have now abdicated in his favour, leaving around 30 contenders in the fray.
Opinion polls have indicated that Karzai is 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 run-off against Abdullah, his former foreign minister.
Ashraf Ghani, a former finance minister, who is running on a campaign of clean governance, job creation and economic development, has criticised Karzai for his alliances with warlords.
Ghani addressed a final rally of 5,000 in the eastern Nangarhar province, pledging to replace the "corrupt government with a legitimate one".
The return overnight from Turkey 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.
|Afghanis are gearing up for elections to be held on August 20 [EPA]
"We must not let it go to a second round and back Hamid Karzai," the heavily protected Dostum told cheering supporters in Shiberghan, his home city.
"There will be a day, God willing, when I can help all of the people of Afghanistan again," said Dostum, who addressed the crowd in Dari, Pashto, Uzbek and Turkmen.
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.
The US embassy in Kabul expressed serious concern to the Afghan government on Monday following Dostum's homecoming, "particularly during these historic elections".
A US official told the Reuters news agency that Washington had made its "serious concerns" about a future role for Dostum clear to the Afghan government and said Dostum's reputation "raised questions of his culpability for massive human rights violations".
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," 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, referring to the name the Taliban gave to the country during its rule.
Officials have said that insecurity could prevent voting in around seven to nine districts across the country.