* Caucasus insurgents have threatened to prevent Olympics
* Critics say measure violates freedom of assembly, movement
* Gay rights activist plans rally on opening day of Games
Russian President Vladimir Putin has signed a decree to tighten security in the Black Sea resort of Sochi during the 2014 Winter Olympics, imposing restrictions on freedom of movement and assembly that critics said violated human rights.
The decree creates a "prohibited zone" encompassing much of the sprawling city, barring travel into Sochi by road without special permission and banning public gatherings "not connected" with the Olympics in high security areas on land and sea.
Putin has staked his reputation on the security and success of Russia's first Olympics since the Soviet era and has ordered law enforcement authorities to prevent any attacks by Islamist militants based in the nearby North Caucasus provinces.
In early July, insurgent leader Doku Umarov revoked a previous order to refrain from attacking Russian targets outside the North Caucasus and urged militants to use "maximum force" to prevent Putin staging the Games.
Published in the official newspaper Rossiiskaya Gazeta on Friday, the decree enters into force on Jan. 7, a month before the opening ceremony, and expires on March 21, a month after the Olympics end.
Human rights activists said the restrictions were draconian.
Barring cars from outside Sochi restricts Russians' freedom of movement and the ban on most public gatherings violates their constitutional right to free assembly, said Pavel Chikov, a human rights lawyer and head of the legal aid group Agora.
He told Ekho Moskvy radio such restrictions required passage of a law or imposition of a state of emergency.
"Under such conditions the Olympics, which are supposed to be celebration of sport and democracy, will become the exact opposite," prominent Russian environmental activist Alexei Yablokov said in a comment posted on the Internet.
"Nature is being destroyed, people are being evicted from their homes, and now it turns out that a state of emergency is being imposed," he said.
Gay rights activist Nikolai Alexeyev said he planned to hold a gay pride march in Sochi on the opening day of the Olympics and that it could not be prohibited by presidential decree.
"Are the Olympics an emergency situation, like a war or a natural disaster?" Alexeyev was quoted as saying by the website grani.ru. "There has been no precedent in history in which peaceful demonstrations have been banned in a city where the Olympics were being held."
A law Putin signed this year banning gay "propaganda" has been criticised by Western governments and prompted calls for a boycott of the Olympics. Russia has offered assurances that the law will not affect athletes or spectators.
In the decree, Putin said its purpose was the "implementation of increased security measures" called for in a law adopted in 2007, when the Olympics were awarded to Sochi.
Kremlin officials could not immediately be reached for comment on Saturday.