Paris intensified security on Thursday after London blasts killed more than 33 people and injuring hundreds of others earlier in the day.
Paris raised the four-level security system (yellow, orange, red, scarlet) to "red" alert, with increased patrols in train and metro stations and police random checks on train passengers.
With a "red" alert, France also closes down parts of its air space, forbids private leisure aircraft to fly, and stocks potable water, said a spokeswoman of the Paris police.
"Now more than ever, our democracies must come together and show that they are united against the terrorist menace," French Prime Minister Dominique de Villepin said in a statement, adding " Now more than ever, we have to show vigilance and determination."
The attacks occurred during morning rush hours in London's subways in the financial district and on a bus in the center of the capital, one day after London beat out Paris to host the 2012 Olympic Games and as G8 summit is being held in Scotland. At least 33 people were killed and 340 others injured.
A previously unknown group which calls itself the Organization of Al Qaeda Jihad in Europe claimed responsibility for the blasts and threatened similar attacks against Italy, Denmark and other " Crusader" states with troops in Iraq and Afghanistan.