At least four people were shot dead and several wounded during a second day of anti-government protests in Guinea's capital Conakry on Friday, an opposition coalition said.

The protests were spurred by fears that Guinea's ruling junta, which took power in a coup last September, is not moving quickly enough to restore civilian rule.

"The FNDC is profoundly shocked and outraged by the loss of human lives registered during the day of Friday 29th of July," the National Front for the Defence of the Constitution (FNDC) said in a statement.

Four deaths were reported by monitoring groups and victims' family members, and several people wounded by gunshot of which five in critical condition, it added.

Authorities have not yet commented on the protests, but a human rights activist working with families to identify victims told Reuters the death toll was credible.

Residents said gunshots rang out in several Conakry neighbourhoods on Friday night as protesters clashed with security forces.

On Thursday, the FNDC registered one death, several wounded and more than 100 arrests.

Police did not confirm that death. A senior police official who declined to be named said that 88 people were injured, included 12 police.

One of the FNDC's leaders, Ibrahima Diallo, told journalists that 102 people had been arrested on Thursday and 23 of them sent to a military camp overnight.

This week's demonstrations were the second major protest against the junta in recent months. One person was killed in a protest over fuel price hikes in June, and Guinea charged a police officer with his murder.

Coup leader and interim President Mamady Doumbouya has proposed a 36-month transition to elections, which regional leaders and opposition politicians have rejected.

Following criticism of the 36-month timeline, the junta banned all public demonstrations several months ago.

Guinea's attorney general ordered a judicial investigation into those responsible for Thursday's march.

The protest was called by the FNDC, a coalition of politicians and activists that also opposed former President Alpha Conde's efforts to hold onto power.

Conde was overthrown last year after changing the constitution to allow himself to run for a third term in 2020.