Many unsuccessful candidates for seats in the Wolesi Jirga, the lower house of parliament, marched with their supporters in Kabul and the eastern cities of Jalalabad and Khost to demand that a vote recount be held and monitored by an international committee.
Demonstrators also accused the Afghan Independent Election Commission of neglecting their allegations of electoral fraud.
Although preliminary results for the September 18 vote were announced, the official final results have yet to be declared.
Protesters in Jalalabad warned that the incoming parliament would face "deadly" confrontations if the authorities failed to address the protesters' grievances.
Jalalabad protester Maulawi Azizpant called the elections "illegal" because of what he called large-scale fraud. He told RFE/RL that if the new parliament was formed on the basis of these corrupt elections it would lack credibility and legitimacy.
Azizpant said that "neither Afghanistan's constitution nor international law recognizes such elections; we will confront any parliament that is the result of vote manipulation. We will cause it to collapse."
Shortly after unveiling preliminary results, the Election Commission annulled more than 1 million ballots that it said were fraudulent.
Former parliament deputy Kabir Ranjbar, who lost his seat in the September elections, said that as opposition to the results of the vote grew, "election commission officials will lose control of the situation."
He added that "it is obvious that opposition [to the election results] is growing."
But an Election Commission spokesman told RFE/RL that the protests organized by the candidates who lost in the elections would have "no considerable effect" on the process or the election results.
Some Afghanistan analysts say that continued opposition or rejection of the results could undermine the credibility of the Afghan government.
They say such events could affect NATO policy toward Afghanistan. The alliance is expected to discuss its long-term partnership with the Afghan government and the transition to Afghan-led security in the country at the NATO Lisbon summit on November 19-20.