* Mild sentences expose authorities' weakness - analyst
* Crowd outside court hails plotters as national heroes
* Canada's Centerra Gold embroiled in Kyrgyz power struggle
* Authorities claiming damages, new contract from Centerra
By Olga Dzyubenko
BISHKEK, March 29 (Reuters) - A court in Kyrgyzstan sentenced three opposition members of parliament to up to 18 months in jail on Friday for leading a protest the judge said had aimed to seize power by force in the Central Asian nation.
Prosecutors had sought jail terms of up to 10 years for nationalist MPs Kamchibek Tashiyev, Sadyr Zhaparov and Talant Mamytov, who led a crowd which tried to storm government headquarters last October.
The protestors were demanding the renationalisation of the huge Kumtor gold mine, long a bone of contention in the impoverished former Soviet Union state of 5.5 million and run by Canadian firm Centerra Gold.
The clashes between police and supporters of the opposition Ata Zhurt party were the most violent in the capital Bishkek since those that deposed President Kurmanbek Bakiyev in 2010 in the second revolt since 2005 in the mainly Muslim state.
Judge Adylbek Subankulov said the three parliamentarians were guilty of plotting to seize power by force and sentenced them to up to 18 months in a high-security prison.
"We only voiced the will of the people to return Kumtor to our nation," Tashiyev, confident and defiant, said from a metal cage in which the three MPs were held before the sentence was pronounced.
Several hundred opposition supporters faced police with truncheons outside, chanting: "Acquittal!" and "Freedom!"
"Freedom to the people's heroes!" read one of the posters held by protesters who included many youths and women.
"The revolutions on March 24 (2005) and April 7 (2010) were most directly linked to the issue of Kumtor - we wanted to attract the attention of the state to this problem and put an end to it," Zhaparov said before the judge.
Local political commentator Alexander Knyazev said the mild sentences showed authorities' concern over the support the three MPs enjoy in the less developed and ethnically mixed south, where the grip of the central government remains tenuous.
President Almazbek Atambayev, whose country borders China and hosts both U.S. and Russian military air bases, was booed by hundreds of Ata Zhurt supporters when he visited the Jalalabad region on Thursday.
"The authorities simply feared that this trial could be used by the opposition to escalate protest actions," Knyazev said.
Under a Bakiyev-era contract drawn up in 2009, the Kyrgyz state is a 33 percent shareholder in Centerra.
Kumtor, located 4,000 metres above sea level in the Tien Shan mountains, is the biggest gold mine operated by a Western-based company in the former Soviet Union.
After ice movement in the pit cut Kumtor's output in 2012, Kyrgyzstan's gross domestic product also fell. The mine with some 3,400 permanent staff and contractors made up 5.5 percent of Kyrgyz GDP and 18.9 percent of industrial output in 2012.
Prime Minister Zhantoro Satybaldiyev, whose coalition government took power last September pledging to alleviate poverty, has repeatedly said that Kumtor should not be nationalised.
But he is now locked in talks with Centerra Gold's management after parliament gave the government in February until June 1 to renegotiate the current financial agreements or rip them up if no compromise is found.
The government wants to levy heavier taxes on the Toronto-listed company and has also sent it two environmental damage claims worth a total of $457 million.
Centerra says such claims are exaggerated or without merit.