French Interior Minister Bernard Cazeneuve warned against jumping to conclusions after the pre-dawn shooting in Montrouge.
The gunman was armed with a machine-gun and a pistol and wore a bullet-proof jacket, police sources told AFP news agency.
A local resident, Ahmed Sassi, described a "scene of panic". He said he had seen a police officer standing and than a man dressed in dark clothes who ran up and shot the officer "at point black range".
"I saw the officer fall and a colleague call for help," Mr Sassi said.
Overnight, seven people believed to be connected to the two main suspects in the Charlie Hebdo attack were detained in the towns of Reims and Charleville-Mezieres, as well as in the Paris area.
Cherif Kouachi was sentenced in 2008 to three years in prison for belonging to a Paris-based group sending jihadist fighters to Iraq.
A third suspect, 18-year-old Hamyd Mourad handed himself in to police in Charleville-Mezieres. He reportedly surrendered after hearing his name on the news.
'We killed Charlie Hebdo'
Paris has been placed on the highest terror alert and extra troops have been deployed to guard media offices, places of worship, transport and other sensitive areas.
Eight journalists - including the magazine's editor - died along with a caretaker and a visitor when masked men armed with assault rifles stormed the Charlie Hebdo offices during an editorial meeting. Eleven people were also wounded, some seriously.
Two policemen were killed on the street outside as the gunmen made their escape by car.
The magazine's office was firebombed in 2011. It had angered some Muslims by printing cartoons of the Prophet Muhammad as part of its irreverent take on news and current affairs.
Witnesses say the gunmen shouted "we have avenged the Prophet Muhammad" and "we killed Charlie Hebdo", as well as "God is Great" in Arabic.
The attackers fled to northern Paris before abandoning their car and hijacking a second one, police say.
Vigils were held through the night in Paris and cities worldwide in tribute to the dead. many demonstrators held up placards reading "Je suis Charlie" (I am Charlie) in solidarity with the victims.
President Hollande said the country's tradition of free speech had been attacked and called on all French people to stand together.
"Today the French Republic as a whole was the target," he said in a televised speech.
Piles of pens - symbolising freedom of expression - and candles were laid across the Place de la Republique square in Paris where thousands of people had gathered.