US President Barack Obama says he does not want to speculate about who is responsible for the multiple attacks that terrorised Paris on Friday evening.
Mr Obama said France has been an "extraordinary terrorism partner" and the US will do "whatever it takes to bring these terrorists to justice".
A state of emergency will be declared," Mr Hollande said. "The second measure will be the closure of national borders," he added.
The Paris municipality asked Parisians in a tweet to stay home. The Mayor of Paris said it was too early to say they were coordinated attacks, but it looked that way.
At least 60 people were killed and 60 wounded in the attacks and a hostage situation, reportedly involving up to 100 people, was under way at a concert hall in the French capital, French media reported on Friday.
British Prime Minister David Cameron expressed solidarity and offered any help needed.
UN Chief Ban Ki-Moon condemned the attacks as "despicable".
NATO Secretary-General Jens Stoltenberg said the Atlantic defence alliance would stand with France "strong and united" against terrorism, following the killing of dozens of people in Paris on Friday night.
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