* New law restricts prayer rooms, missionaries
* Freedom House says law is "repressive"
* Concerns about spread of Islamist militancy (Adds OSCE statement, paragraphs 10-11)
ASTANA, Sept 29 - Kazakhstan's Senate approved on Thursday tougher laws on religious activity in the Central Asian state, ignoring criticism of its response to what it calls the growing threat of extremism.
The new law, which will ban prayer rooms in state institutions, will have to be signed into law by President Nursultan Nazarbayev. Kazakhstan's veteran leader proposed tough new laws to his compliant legislature a month ago.
Kazakhstan, where 70 percent of the 16.5-million population is Muslim, has only recently witnessed outbursts of militant Islam experienced by other former Soviet states in the vast region bordering Afghanistan.
Last month's detention of a group of religious extremists planning "acts of terror" unsettled many in Kazakhstan, an oil-rich country ruled by Nazarbayev for more than two decades.
Kazakhstan also last month temporarily blocked access to a number of foreign Internet sites after a court ruled they were propagating terrorism and inciting religious hatred. A suicide bomber blew himself up in the city of Aktobe in May.
The new law, which has stirred debate in officially secular Kazakhstan, stresses "the historic role of the Hanafi school of Islam and of the Christian Orthodox faith in the cultural and spiritual development of the Kazakh nation".
The vast majority of Kazakhstan's Muslims are followers of the Hanafi school of law, considered to be the oldest and most liberal within the Sunni Muslim tradition.
The law also requires the review of all religious literature and the mandatory annual registration of all foreign missionaries, who can be expelled if deemed to pose a threat to the "constitutional order and public peace".
Authorities say they want to stop the spread of extremism into Kazakhstan, the most prosperous of Central Asia's nations, from the overpopulated and impoverished Ferghana Valley shared by ex-Soviet neighbours Kyrgyzstan, Uzbekistan and Tajikistan.
The Organisation for Security and Cooperation in Europe (OSCE), Europe's main rights and security watchdog, criticised the planned measures and urged a full review.
"The new law appears to unnecessarily restrict the freedom of religion or belief," said Janez Lenarcic, the OSCE's human rights chief. Kazakhstan is one of the OSCE's 56 member states.
U.S.-based watchdog Freedom House had called for Kazakhstan's senate to reject the new law, describing it as "repressive".
"These provisions are very troubling, as they grossly curb Kazakhstani citizens' right to freely practice and express their faith," Susan Corke, Freedom House's senior programme manager for Eurasia, said in a statement issued on Sept. 22.
A day earlier, Kazakhstan's lower house of parliament had voted in favour of the bill.
"This latest piece of legislation signals the continuing deterioration in the country's human rights and religious freedom situation," Corke said.
Kairat Lama Sharif, head of the government's Religions Agency, said many other countries had also taken steps to curb the spread of extremist ideology.
Asked to respond to the Freedom House statement, he said: "Many Western countries are making their own laws. Three states have already introduced laws to ban the hijab in their countries."
Kazakhstan has no official restrictions on wearing the hijab or other forms of religious dress. (Reporting by Raushan Nurshayeva; Writing by Robin Paxton; editing by Elizabeth Piper, Karolina Tagaris)