More than 2,000 protesters came out on Saturday, some brandishing placards inscribed with slogans such as "respect for the peoples vote" and waving UNITA flags as they marched in downtown Luanda.
The demonstrations were called by the National Union for the Total Independence of Angola (UNITA) - the largest opposition party.
Under its charismatic leader Adalberto Costa Junior, 60, UNITA has proved popular in urban areas and among young voters eager for economic change.
UNITA, a former rebel movement which fought a 27-year civil war against the MPLA government has rejected the outcome of the August 24 vote.
It challenged the outcome in court, but the country's top court dismissed the petition.
Other opposition parties and civic groups had said the vote was marred by irregularities.
The formerly Marxist Popular Movement for the Liberation of Angola (MPLA) won a wafer-thin majority of 51.17 percent of the vote, handing President Joao Lourenco, the 68-year-old former general, a second term in power.
It was the MPLA's worst electoral showing since independence from Portugal in 1975.
"The MPLA must understand that there are other voices they have to listen to," Maria Saraiva, a 33-year-old unemployed hairdresser told AFP at the start of the march.
"Today is the first of many steps we as UNITA sympathizers will take to force political changes," said sound engineer Jose Costa, 46.
Andre Bastos, a 27-year-old law student at Agostinho Neto University, said: "I am here... to add my voice to those who continue to say the election results were stolen".