PARIS—Heads of state from some 150 countries began arriving in Paris Sunday for an international climate conference, triggering a lockdown of a city already on high alert after the Nov. 13 terror attacks.

The police closed major routes Sunday afternoon between airports, downtown Paris and the Le Bourget conference center where the conference known as COP21 will take place. Authorities told Parisians to not take to their cars and avoid using public transport through to the end of Monday, when the main routes will be closed for the whole day.

The terror attacks that left 130 dead have chilled the atmosphere in the French capital ahead of the climate conference after over two years of careful diplomacy to prepare a global agreement aimed at limiting the rise in temperatures.

Using powers under a state of emergency, French authorities have banned public gatherings in Paris that climate activists hoped would be leave a legacy from the city's hosting of the conference.

"It is traumatic and a real handicap that only those already involved will be able to express themselves," said Nicolas Hulot, a climate activist and close adviser to the government in his role as special envoy for the environment. "But I'm well placed to know that we have excellent reasons to ban these marches."

Still, thousands flouted the ban on protests and formed a human chain to call on leaders to do more to fight climate change. The chain spanned an area near where gunmen and suicide bombers unleashed carnage just over two weeks ago.

Later, riot police fired tear gas as they clashed with demonstrators who refused to disperse from Place de la Republique, north east of central Paris. News channels showed pictures of candles and flowers from a memorial to the victims of Paris attacks scattered after the confrontation. Police said 174 demonstrators were detained.

With two police still hunting two men they suspect of involvement in the Paris attacks, French authorities have mobilized 2,800 police officers at the Le Bourget site where the 150 world leaders will address the COP 21 conference one after another Monday. France has also tightened security on a national level by deploying 8,000 gendarmes and police officers to check papers at borders that are normally open.

"With a very high threat against our country, the success of the COP 21 also depends on an optimal security," the interior ministry said.

The speeches by world leaders will kick off nearly two weeks of talks, during which diplomats will try to overcome obstacles to getting nearly 200 countries to sign up to a deal to try to limit global warming.

In the build up to the talks in Paris, countries have already published commitments to limit emissions of greenhouse gases. But experts say that the sum of the pledges will fall short of limiting a rise in temperatures that would cause irreversible damage to the planet.

Negotiators are now trying to find an agreement to revise pledges up every five years to meet the targets in the future. But some countries are hesitant to legally bind themselves to commitments that could limit their future economic development.

"We must convince all countries so that there is an ambitious agreement, which means a binding agreement," French President Franç ois Hollande said Sunday. "If there are no binding elements in the agreement, there won't be any credibility."

Noemie Bisserbe contributed to this article.

Write to William Horobin at William.Horobin@wsj.com

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(END) Dow Jones Newswires

November 29, 2015 20:25 ET (01:25 GMT)