Hundreds of supporters of the former Brazilian president, Luiz Inácio Lula da Silva, have set up camp outside the cell where he is being held.
Lula has been sentenced to 12 years and one month in prison for corruption and money laundering.
He handed himself in to federal police on Saturday in São Paulo and was flown to the southern city of Curitiba.
The former leader says he will continue to fight against his conviction, which he says was politically motivated.
Polls conducted before he was jailed suggested that Lula, who governed from January 2003 to December 2010, was the frontrunner in October's presidential election but his imprisonment has left the race wide open.
As a convict Lula would normally be barred from standing for election in October, but Brazil's top electoral court will make the final decision if and when he submits his candidacy.
Lula's routine in jail for the first 10 days of settling-in
- Morning coffee with bread and butter
- Lunch and dinner tray with plastic utensils
- No outside food allowed during the settling-in period
- No visits except for those from his lawyers in this 10-day period
- Allowed into the prison courtyard for two hours a day
- Cell measures 15 sq m (160 sq ft) with access to private toilet and shower
Supporters of the former leader say they will not budge from the "Free Lula" camp until he is released.
Security around the federal police building in Curitiba where he is being held was strengthened after some of Lula's supporters clashed with police on Saturday night. Nine people were injured.
Police estimated the numbers of people camping around the building on Sunday afternoon at about 700, with more due to arrive.
The leader of the Workers' Party, Gleisi Hoffmann, said Lula was "a political prisoner" and that his party would not give up the fight to have him released.
Ms Hoffman said she hoped his release could come as early as Wednesday, when the Supreme Court could revisit its decision, taken only last Thursday, that defendants whose first appeal has failed can be jailed.
The ruling on Thursday was close, with six Supreme Court Justice in favour of jailing Lula and five against.
It would only take one of the justices to change their mind for Lula to be released to pursue his appeals against his conviction, which could take months if not years.
However, if the appeals were to go against him, he would still face jail eventually.
Lula is still also facing six separate pending trials for corruption.