SAO PAULO (AFP) - Brazil's ex-president Luiz Inacio Lula da Silva was headed back to prison on Saturday (March 2) after being allowed to attend the funeral of a grandson who died suddenly at age seven.

Early in the day, the popular leftist leader stepped out of his cell in Curitiba - where he is serving two concurrent 12-year sentences for corruption - and then flew in a small plane to Sao Paulo, some 400km to the north-east.

A crowd at the Sao Bernardo do Campo cemetery in a Sao Paulo suburb greeted Lula warmly upon his arrival for the service, shouting "Free Lula" and " Lula, warrior for the Brazilian people."

Young Arthur Araujo Lula da Silva, whose father Sandro is one of Lula's five sons, was felled by a sudden bout of meningitis.

As dozens of armed military police surrounded the area, the crowd of sympathisers at the cemetery numbered perhaps 500, including Lula's successor Dilma Rousseff and Fernando Haddad, the Workers' Party candidate for president in 2018. Hundreds of supporters wore the red colour of Lula's Workers' Party.

At the crematorium room where close friends gathered before Arthur's open coffin - and where Lula made brief remarks - there were flower wreaths from Lula's political and trade union allies, even one from Venezuela's embattled socialist president Nicolas Maduro, the daily Folha de S. Paulo reported.

As Lula left two hours later, a grim expression on his face, he waved briefly to supporters and somberly shook a few hands before being hustled away by military police for the return back to jail.

A federal court had ruled late Friday that the 73-year-old Lula could attend the funeral service.

'A high-profile thug'

Arthur, who had twice visited his grandfather in his cell in Curitiba, died on Friday in a Sao Paulo hospital.

Gleisi Hoffmann, the Workers' Party leader, visited Lula in prison after he learned of Arthur's death and said the ageing leftist was "downcast."

"He cried several times and we tried to console him," she said.

While news of Arthur's death provoked sympathetic messages on social media - including from a former political foe of Lula's, the head of the National Assembly Rodrigo Maia - a son of President Jair Bolsonaro sparked a controversy by criticizing Lula's release.

" Lula is a common-law prisoner," Eduardo Bolsonaro, a deputy in his father's Social Liberal Party, said on Twitter.

"When a relative of another prisoner dies, is he escorted by the federal police to go to the services?"

He called the temporary release "absurd," adding, "It only allows a high-profile thug to pass himself off as a victim."

That remark prompted an angry social media backlash, which prompted a more conciliatory tweet from the younger Bolsonaro.

Politics aside, he said, the death of a child is "dreadful."

His father, the president, has made no comment on the matter. During his electoral campaign in 2018, he said he hoped Lula would "rot in prison."

Lula, who was president from 2003 to 2010, has consistently denied the corruption charges for which he was jailed, saying he was the victim of political machinations.