Just weeks ago, the federal government decreed that Brazil's army would take over all security operations through the end of the year in Rio, where murders have risen sharply. Franco, part of a commission to oversee the military intervention, harshly criticised the move on Sunday, saying it could worsen police violence against residents.

"It is far too soon to say, but we are obviously looking at this as a murder in response to her political work, that is a main theory," said a Rio de Janeiro public prosecutor, who spoke on condition that he not be named as he was not authorised to discuss the case.

Rivaldo Barbosa, head of Rio's Civil Police, told reporters that "one of the possibilities in analysis is, yes, an execution." He did not speculate on who may have been responsible.

An investigator with the city's police force went further, saying the prime motive appeared to be Franco's calling out police for allegedly killing innocents in their constant battles with drug gangs.

Political violence is common in Brazil - but typically in smaller or more impoverished cities.

"This is certainly an attack on the rule of law and an attack on democracy," Temer said in a video published on social media. He delayed a previously-scheduled event at the presidential palace to hold a cabinet meeting on Rio's security, and said the federal police was willing to help with the investigations.

Her violent death underscores challenges in reducing bloodshed in Brazilian cities, many of which rank among the most violent in the world. Violence is particularly intense in Rio de Janeiro, where criminals frequently clash with police forces, causing the shutdown of major motorways and disrupting the local economy. Public order has become a central theme ahead of this October's presidential election.

A day before she was killed, Franco asked on Twitter how many more deaths would be necessary for the situation to improve. Commenting on the death of a youth, she said: "One more homicide of a young man that may be put towards the police count. Mathews Melo was leaving church. How many more need to die for this war to end?"

On Sunday on her Facebook page, Franco also decried what she alleged to be the police killing of two boys during a police raid in an area called Acari.

"We must scream out so that all know what is happening in Acari right now. Rio's police are terrorising and violating those who live in Acari," Franco wrote. "This week two youth were killed and tossed into a ditch. Today, the police were in the street threatening those who live there. This has been going on forever and will only be worse with a military intervention."

In a Sunday statement to the O Dia newspaper, police said they carried out an operation in the area, were fired upon by drug traffickers, returned fire but had no knowledge of any deaths.

Thousands gathered to watch Franco's coffin enter the state assembly building in downtown Rio on Thursday afternoon, with local media providing blanket coverage of the cortege. As night fell, huge crowds marched in central Rio and Sao Paulo in protest at her murder.

Public figures from across Brazil's political spectrum expressed outrage over the killing. Finance Minister Henrique Meirelles, who is contemplating a bid for the presidency, said the assassination underscored the need of the Rio military operation. "Those who did this need to be punished," Meirelles said.

Members of Franco's family and her party were unaware of any threats against her life, according to local media.

In the months before the 2016 city council elections in Baixada Fluminense, a hardscrabble region the size of Denmark that surrounds Rio, at least 13 politicians or candidates were murdered before ballots were cast.

Franco, who was raised and lived in the Mare complex of slums, long one of Rio's more dangerous areas, received over 46,500 votes in the 2016 election, the fifth most popular representative out of 51 council members.

Bloomberg, Reuters