LE BOURGET, France – Addressing the twin threats of global warming and extremist violence, the largest group of world leaders ever to stand together kicked off two weeks of high-stakes climate talks outside Paris on Monday, saying that by striking an ambitious deal to cut emissions they can show terrorists what countries can achieve when they are united.

The gathering of 151 heads of state and government comes at a somber time for France, two weeks after militants linked to the Islamic State group killed 130 people around Paris. Fears of more attacks have prompted extra-high security and a crackdown on environmental protests – and threaten to eclipse longer-term concerns about rising seas and increasingly extreme weather linked to man-made global warming.

“The challenge of an international meeting has never been so great because it’s the future of the planet, the future of life,” French President Francois Hollande said after a moment of silence for attack victims in France, Lebanon, Iraq, Tunisia and Mali.

“There are two big global challenges that we must face,” he added, urging leaders to create a world free from both environmental destruction and extremist violence.

Many of the leaders paid their respects at sites linked to the attacks on their way to the conference. President Barack Obama, in a late-night visit, placed a single flower outside the concert hall where dozens were killed, and bowed his head in silence.

“We stand with Paris,” said U.N. climate change agency chief Christina Figueres said at talks near Le Bourget airfield, just north of the city. “The city of light, now more than ever, is a beacon of hope for the world.”

On Sunday, hundreds of thousands of people rallied around the world, calling on the leaders to make real progress at the talks. Violence erupted after one peaceful demonstration in Paris, and hundreds of people were arrested. U.N. Secretary-General Ban Ki-moon took note of the protests in his opening remarks.

“The future of the people of the world, the future of our planet, is in your hands,” Ban told negotiators. “We cannot afford indecision, half measures or merely gradual approaches. Our goal must be a transformation.”