Brazil trial begins over murder of iconic activist Franco
Ronnie Lessa and Elcio Queiroz, both former military police officers, have already admitted to killing Franco and her driver in a drive-by shooting on March 14, 2018.
Franco, who grew up in a Rio slum and was an outspoken critic of police brutality, was 38 at the time of her death.
"Today is the first step towards justice being served. We must not trivialize the loss of the lives that were taken from us," her daughter Luyara Santos, who was 18 at the time of the killing, told a rally outside the courthouse on Wednesday.
"After all this time I still feel as I did on the day my daughter was taken from me," Franco's mother Marinete Silva, told the gathering.
She was joined by her daughter, Marielle's sister Anielle Franco, who is Brazil's current minister for racial equality.
Lessa has confessed to firing on Franco's car with a machine gun, while Queiroz has confessed to being the driver during the attack.
The pair appeared in court by video link-up from prison.
Prosecutors are seeking the maximum sentence of 84 years imprisonment for each.
Authorities believe organized crime and militias played a role in Franco's assassination.
A congressman Chiquinho Brazao and his brother Domingos Brazao, an advisor with the state auditor, were arrested in March and accused of being among the masterminds of the attack.
Besides campaigning for the rights of young black Brazilians, women and members of the LGBT community, Franco had frequently denounced abuses carried out by powerful militias in Rio's impoverished favelas.
Among the witnesses who will be called to testify is Franco's former aide Fernanda Chaves, who survived the attack.
Icon
After her death, Franco became an icon of the left and the fight against racism in Brazil.
"Being here is an act of resistance. As a black woman I must be present to make my voice heard and show important Marielle and Anderson were and still our in our lives," Geovanna Januario, a 26-year-old geographer, who is black, told AFP outside the courthouse.
Lessa led police to the Brazao brothers, saying that the pair had offered him a lucrative payoff to kill Franco on behalf of militias.
The brothers deny any involvement in the attack.
Last week, they were questioned by the Supreme Court, as was former Rio police chief Rivaldo Barbosa, who is accused of obstructing the investigation into Franco's death.
Barbosa also denies the allegations.