* Putin's amnesty could free Pussy Riot members
* Activists say plan, due to pass on Wednesday, too narrow
* Text may change to allow Greenpeace activists' release
By Steve Gutterman
MOSCOW, Dec 17 (Reuters) - A prison amnesty that President Vladimir Putin has portrayed as an act of mercy and could free members of punk band Pussy riot came a step closer on Tuesday when Russian lawmakers gave it preliminary approval.
Putin submitted his proposal last week for the amnesty to mark the 20th anniversary of Russia's post-Soviet constitution, after long debate about how many it would free and whether it would apply to inmates his critics call political prisoners.
Rights activists say will free only a small fraction of the country's more than half a million inmates.
The proposal would leave Putin foes such as former oil tycoon Mikhail Khodorkovsky in jail and will not benefit Alexei Navalny, an opposition leader serving a five-year suspended sentence on a theft conviction he says is politically motivated.
Approved in a initial vote by the lower house of parliament, it could, however, lead to the early release of two Pussy Riot members who are due to be freed in March after two years in jail for an anti-Putin protest in Russia's main cathedral.
Analysts say the Kremlin may believe freeing prisoners such as Nadezhda Tolokonnikova and Maria Alyokhina, whose punishment has been condemned in the West as excessive, could ease criticism before Russia hosts the Winter Olympics in February.
Environmental group Greenpeace has said that under the current wording, the amnesty would be unlikely to benefit 30 people from 18 nations who face trial over a September protest against Russian oil drilling in the Arctic.
But Russian media reported that before a final vote expected by the pro-Kremlin dominated Duma on Wednesday, the text could be changed in a way that would end their prosecution, which has also drawn criticism from the West.
Even if they are freed, rights activists estimate the amnesty will lead to the release of fewer than 1,500 convicts out of a prison population of about 680,000 - far fewer than a plan backed by members of Putin's own human rights council.
Out of some 30 people on trial or facing prosecution over a protest on the eve of Putin's inauguration to a third presidential term last year, it is likely to benefit only those few who are not accused of violence against police.
"This amnesty is not as broad as it may seem. The draft would not apply to ... someone who tore a button off a policeman's uniform or shoved him," said Communist lawmaker Yuri Sidelnikov. "That is far from the principle of mercy."
Putin, who critics accuse of curbing democracy over 14 years in power and clamping down on dissent during his third presidential term, denies such accusations and said last month that the amnesty should "underscore the humanism of our state".
It would free many elderly and very young inmates, as well as women with young children.
(Additional reporting by Maria Tsvetkova; Editing by Alison Williams)