Reports in France suggest that as many as 100 hostages were killed this evening in the Bataclan Concert Hall.
Security forces launched an assault on the the concert hall where one official described “carnage” inside the building, saying the attackers had tossed explosives at the hostages.
More than 35 people are feared dead across Paris in at least six attacks.
A French police union official said there were two suicide attacks and a bombing near the Stade de France stadium, where the national side was playing Germany in a friendly football match.
French president Francois Hollande has declared a state of emergency and closed the French borders after the attacks across Paris this evening.
The attacks in Paris come in a year which has seen France gripped by the threat of terrorism.
Tonight’s developments brought back memories of January, when one of the world’s great cities was brought to a standstill by Islamist extremists.
On that occasion the French capital was rocked first by the Charlie Hebdo atrocity, when 12 people were killed after gunmen stormed the offices of the satirical magazine.
The sense of panic heightened when there was a subsequent attack on a Kosher supermarket, and the incidents triggered worldwide outrage.
Since then there have been a number of more minor strikes or attempts. In one, three Americans and a Briton overpowered a heavily armed gunman on a train from Amsterdam to Paris.
The horrific scenes in the city tonight represent an escalation compared to the previous incidents.
It is believed that the reported death toll means it is the deadliest atrocity on French soil in recent times. In 1961 a train bombing killed 28 people.
Other bloody attacks in Europe have included the 7/7 attacks in London, in which 52 people were killed, in 2005.
The year before a series of explosions in Madrid killed 191 people.
The reports of mass casualty shootings is likely to increase the sense of alert in other cities, including London.
Security services and senior police in the UK have repeatedly highlighted the risk of a Mumbai-style roaming gun massacre.
Earlier this year police carried out a simulated terror attack in the capital to test the emergency response to such a strike.