Abdullah has vowed to reject the election result, saying he was the victim of massive ballot-box stuffing in the June 14 poll, while his rival Ashraf Ghani has claimed victory by more than one million votes.
As tensions rise between opposing supporters, the United Nations has expressed fears of spiralling ethnic violence at the same time as US-led troops pull out after their 13-year war against Taliban insurgents.
"Death to Ashraf Ghani! Death to the election commission!" Abdullah's supporters chanted near the presidential palace as he was carried through the rowdy crowds on the roof of a truck.
"If our demands are not met, we will continue our protest as long as fraud votes are not separated from clean votes," said Ahmad Zia, 22, waving an Abdullah poster and the Afghan national flag.
More From This Section
Shaperai, 55, one of the few female protesters, said: "The government and the election commission have stolen our votes. They are traitors and cheats."
Abdullah went into the second-round run-off election as the favourite to succeed President Hamid Karzai, who came to power after the Taliban regime was ousted by a US-led offensive in 2001.
Ghani put his improved performance down to targeting women, workers, farmers and other social groups, rather than relying on ethnic loyalties.