A gunman has killed a policewoman in Paris, a day after suspected Islamists killed 12 people at the office of a satirical magazine.

A second person was seriously injured in the attack in the southern suburb of Montrouge, after which the gunman fled.

It is unclear of the incident is related to the Charlie Hebdo massacre, which traumatised France.

Police have made seven arrests in the hunt for two brothers, the main suspects,. A third has surrendered.

Arrest warrants were issued for Cherif and Said Kouachi, said to be "armed and dangerous".

Sources close to the investigation told AFP news agency of a report that the two had been spotted on Thursday in a car on a road in the Aisne region of northern France.

They were reportedly recognised by the manager of a service station near the town of Villers-Cotteret, and still armed.

A minute's silence was observed at midday (11:00 GMT) across the country as France observed national mourning for the victims of the Charlie Hebdo attack. The bells of Notre Dame cathedral in the capital tolled in mourning.

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.

Two policemen were also killed. 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.

Pencil tributes

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.

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.

Cartoon tributes are circulating on social media, sending out the message of press freedom. One Dutch cartoon plays on 9/11 Twin Towers imagery,showing a plane flying towards two upright pencils.

Thursday's national day of mourning is only the fifth held in France in the past 50 years.