In one incident, supporters gathered to welcome Diallo after he returned from campaigning were caught in a melee of rock throwing and tear gas, witnesses said.
In another suburb, a group of women gathered at the headquarters of a political party that supports Diallo, also were dispersed by police. Diallo has threatened to boycott Sunday's vote unless the head of the national independent electoral commission, who allegedly favors rival Alpha Conde, resigns.
Diallo, a former prime minister, officially won 43% of votes in June's first round, while Conde took 18%.
Meanwhile, one year after the massacre of 157 people during a peaceful rally in Conakry, no army official suspected of taking part in the killings has been brought to court in Guinea, which is still split by political divisions.
Both the United Nations and the International Criminal Court said acts committed on that day constituted crimes against humanity.