A further 8000 police will guard French borders. "Mobile units, riot squads and gendarmerie units will all be called on at an unprecedented level," said Interior Minister Bernard Cazeneuve.
Paris Mayor Anne Hidalgo urged Parisians not to use their cars today or tomorrow when much of the city will be virtually impassable as world leaders host bilateral meetings in hotels and embassies.
The summit, which hopes to secure a deal to hold global warming to a 2C rise, opens to a warning by World Bank chiefs that climate change destabilises countries and allows terrorism and conflict to flourish.
"We are quite certain that the impact of climate change will be to destabilise countries," said World Bank President Jim Yong Kim.
The bank claimed terrorist movements such as Boko Haram in Nigeria and al-Shabaab in Somalia have taken root because of long droughts and growing competition for scarce resources.
"Whenever there is a scarcity of water, when there is erosion of soil, people are competing for these scarce resources. That is the main source of conflict," said Makhtar Diop, the World Bank's vice-president for Africa. "These disputes have spawned instability that has allowed movements such as Boko Haram to take root."
Prince Charles will give the keynote speech at the summit of 195 countries and is expected to warn there is little time to avoid climate catastrophe.
The state of emergency in France, which is due to last three months, has resulted in hundreds of events around the conference being called off. However, some demonstrators are expected to defy police in the name of free speech. Nine French climate activists have been placed under house arrest, accused of flouting a ban on organising protests.
Naomi Klein, the Canadian writer and climate change campaigner, accused French authorities of "a gross abuse of power that risks turning the summit into a farce".
"The French Government, under cover of anti-terrorism laws, [is] shamefully banning peaceful demonstrations and using emergency powers to pre-emptively detain key activists," Klein said.
Greenpeace said people would make themselves heard, whatever the obstacles.
UN bodies, international charities and academics have urged governments to act boldly. "Climate change is the defining issue for the 21st century. We estimate that it is already causing tens of thousands of deaths every year - from shifting patterns of disease, from extreme weather events, and from the degradation of air quality, food and water supplies and sanitation," said a spokesman for the World Health Organisation.
Mark Goldring, the UK chief executive of Oxfam, warned that developing countries' economies faced losing US$1.7 trillion ($2.6 trillion) annually by the middle of the century if global average temperatures rise by 3C.
Five reasons to be cheerful
1. The world really wants a strong deal and this time will get it
There is a universal will to limit emissions, governments understand the science and know that doing nothing is no longer a political or moral option. Evidence of climate change has grown since the Copenhagen summit in 2009, and 2015 has already been declared the hottest year on record. The threat from climate change is also much better understood by the public and this gives politicians the legitimacy to be bold. Non-governmental groups have created a sense of destiny about the Paris talks, pressing the idea that this is the last chance for the world to act to avoid catastrophic change and a deal is certain.
2. A green economy makes financial sense
A bold new international deal committing all countries to reduce emissions is in everyone's long-term economic interests. It will signal to business that governments are legally committed to reducing emissions and will give the private sector and banks the long-term confidence they need to invest in renewable energies and conservation. It should steer financiers, technologists and others away from extracting oil, gas and coal, and towards the development of clean energy. Should large carbon markets also emerge, as big business and the UN want, and should rich countries make good on their pledge to mobilise US$100 billion ($152 billion) a year for poor countries to adapt, then the long-promised global "green economy" should grow fast, benefiting everyone.
3. Nations are ready to commit to real change
Most countries have already stated their intentions. More than 180 countries, representing 90 per cent of global emissions, have submitted their national plans to cut emissions. This is the first time since climate negotiations started 20 years ago that nearly all the world's nations have committed to being part of the solution.
4. The text is manageable
The chances of diplomatic success are much higher than in Copenhagen in 2009, which was billed as the finale of years of talks but ended in diplomatic chaos. The text negotiators and politicians from 195 countries will haggle over is shorter and more focused and many difficult decisions have already been made. The positions of the main emitting countries, such as the US and China, are closer to each other than in the past, so it should be easier to reach a compromise.
5. We're all in it
togetherThe recent Paris terrorist attacks will galvanise the 143-plus world leaders to make a global statement of solidarity and provide the political impetus to secure a strong deal. No country will want to be identified as the one that stopped a deal.
Five reasons to be fearful
1. Countries may not make the necessary compromises
Prince Charles and more than 140 presidents, prime ministers and heads of state will make short, bland statements tomorrow about the need to act, after which negotiators and politicians will have only a few working days to reach diplomatic agreement. There is little chance the gaps between countries can be closed in a few days. So the only way any deal can be reached is if the UN and France, as the hosts, bludgeon through a least-worst agreement over the heads of the many.
2. The fat lady has already sung
The cuts that 180 countries have said they are prepared to make up to 2030 will hold global temperatures only to a 2.7C rise. The absolute maximum rise, if catastrophic warming by the end of the century is to be averted, is thought to be 2C.
3. Who will bear the biggest burden?
Countries are still split on key issues such as reducing emissions, finance and technology. With so many major differences, it will take a heroic effort to reach any deal. The most important hurdle could be over whether industrialised countries should be obliged to cut more than developing countries.
4. Where's the money?
Many of the ambitious plans to cut emissions depend on up to US$1 trillion being made available to invest in renewable energy, farming and forestry. This will depend on flows from new carbon markets and other risky financial sources. In addition, only US$57 billion of the US$100 billion pledged to be "mobilised" by rich countries to help poor countries adapt to a warming world has been identified. Developing countries are not going to roll over without guarantees.
5. Short-term self-interest could win out
There is a genuine will to tackle climate change but not at any price, and many rich countries delude themselves if they think climate change and reducing emissions is a high priority for everyone. These talks have been going on for many years and there is still a deep distrust of the way the US and others have avoided having to change their lifestyles but have bullied poor countries to shoulder the burden of cuts. Many countries resent this and want to determine their own path.
- Observer
