AFP/Paris

Security fears in the wake of Friday's brutal slaying of 129 people in Paris threaten to overshadow a crunch climate summit to be launched by 120 world leaders in the French capital on November 30.

France's government has said it will not "give in" to terrorism and insists that the long-anticipated conference will go ahead, tasked with no less than producing a plan to rescue Earth's climate.

But Prime Minister Manuel Valls said on Monday that "without a doubt" concerts and other gatherings of a "festive" nature would be cancelled.

The UN climate conference, for which about 40,000 delegates, journalists, observers, NGOs and other participants are accredited, will be "limited to negotiation", said Valls -- excluding certain planned side-events.

US President Barack Obama has said he still intends to attend the summit, and Valls said none of the 120-odd heads of state or government who accepted invitations to the opening had asked for a postponement.

"All want to be there. To do otherwise would, I believe, be to yield to terrorism," said Valls, who on Sunday called the gathering "an essential meeting for humanity."

But the violent events of the weekend, claimed by Islamic State jihadists, have thrown into doubt a mass rally in central Paris planned for November 29, on the summit's eve, and another on December 12, the day after the meeting is scheduled to close.

Coalition Climate 21, the civil society grouping organising the marches, was to meet in Paris on Monday to decide how to proceed.

"We are horrified by the attacks, and our heart goes out to everyone in Paris and across the world who is mourning today," it said in a statement.

Valls said the safety of demonstrators was paramount, and security forces would have to "concentrate on the essential" -- the conference itself.

This threatened a series of exhibitions, concerts and other gatherings organised around the city to beat the drum for urgent climate action.

In the midst of a national state of emergency and massive anti-terror deployment, it might be hard to free up the 5,000-odd police and military police required to secure the November 29 rally, a security source told AFP.

The march is meant to start at Place de la Republique square, very close to the scene of Friday's restaurant and bar shootings.

- Security 'reinforced' -

Before the coordinated wave of attacks carried out by three groups of gunmen and suicide bombers on Friday night, it had been announced that 1,500 police, military police and firefighters, more than 100 UN guards and 300 private security agents would secure Le Bourget, the airport outside Paris that will host the conference.

On Sunday, Foreign Minister Laurent Fabius said security would be further "reinforced".

Dubbed COP21 -- for the 21st Conference of Parties to the UN's climate convention -- the gathering aims to deliver the first truly global agreement on reining in greenhouse gas emissions blamed for dangerous levels of climate change.

"Of course COP21 proceeds as planned. Even more so now," tweeted UN climate chief Christiana Figueres.

The conference is meant to crown six years of tough negotiations after talks broke down during the previous attempt at clinching a global deal, in Copenhagen in 2009.

"We are at a crucial point in the fight against climate change: we are three weeks from an agreement," said Matthieu Orphelin, a spokesman for French environment NGO Fondation Nicolas Hulot, which closely follows the talks.

"The COP and Le Bourget will be highly secured. Now we have to think about everything on the fringes."

Even before the attacks claimed by Islamic State jihadists, France had reintroduced border checks as it tightened security ahead of the summit.