Four people were injured, two seriously, in a knife attack in Paris on Friday outside the former offices of French satirical magazine Charlie Hebdo, Prime Minister Jean Castex said.

Two of the victims were in critical condition, the Paris police department said, adding that two suspects were on the run. Police later said one suspect had been detained after the attack.

French media BFM TV and Le Figaro said earlier the suspect had been arrested on the steps of the Paris Bastille opera house.

Schoolchildren were being kept inside in area around the attack, a city hall official said.

“A serious event has taken place in Paris,” said Castex, who was addressing reporters at the time and cut short a visit to northern Paris to head instead to the crisis centre of the interior ministry.

“Four people have been wounded and it seems that two are in serious condition,” he said.

He added the attack had taken place “in front of” the weekly's former offices in the 11th district of central Paris. The magazine's current address is kept secret for security reasons.

The stabbing came as a trial was underway in the capital for alleged accomplices of the authors of the January 2015 attack on Charlie Hebdo.

Twelve people, including some of France's most celebrated cartoonists, were killed in the attack by brothers Said and Cherif Kouachi and claimed by a branch of Al Qaeda.

A female police officer was killed a day later, followed the next day by the killing of four men in a hostage-taking at a Jewish supermarket by gunman Amedy Coulibaly.

The 14 defendants stand accused of having aided and abetted the perpetrators of the 2015 attacks, who were themselves killed in the wake of the massacres.

The magazine had marked the start of the trial by republishing hugely controversial cartoons of Prophet Mohammed (PBUH) that had angered Muslims around the world.

Al-Qaeda then threatened Charlie Hebdo with a repeat of the 2015 massacre of its staff.

The trial in Paris had resumed on Friday after a suspect's coronavirus test came back negative.

The hearing for the fourteen suspects, which opened on September 2, was postponed on Thursday after Nezar Mickael Pastor Alwatik fell ill in the stand.

His lawyer, Marie Dose, said her client had suffered from “a lot of fever, coughing, vomiting and headaches”.

He was back in the box on Friday, after the presiding judge informed defence and prosecution lawyers by SMS late Thursday that the test results allowed for the trial to go ahead.