Former Yukos chief Mikhail Khodorkovsky has been jailed for nine years after being found guilty of six charges including tax evasion.
Mr Khodorkovsky was found guilty at the end of the 11-month trial
But his ordeal may not be at an end as Russian prosecutors have said they will soon bring fresh charges against him.
The news came as Mr Khodorkovsky was found guilty of six of the seven charges of tax evasion, fraud and embezzlement he had faced.
Meanwhile, his lawyers said he would take his appeal to Europe.
As well as appealing against the ruling at Russia's Supreme Court, Mr Khordokovsky will take the case to the European Court of Human Rights, they added.
US President George W Bush said that he had expressed concerns to Moscow that Mr Khodorkovsky was judged guilty before even standing trial.
"As I explained to him (Putin), here you're innocent until proven guilty, and it appeared to us - at least people in my administration - that it looked like he had been judged guilty prior to (having) a fair trial," President Bush said.
He added that the US would watch the case as it continued.
Mr Khodorkovsky is actually expected to serve seven-and-a-half years in jail as the court said his prison term should be reduced because of the amount of time served on remand since his arrest in 2003.
He will serve his time at a medium security prison camp.
But in a statement read out by his lawyer he pledged to continue his public activities behind bars.
It added he would set up "charitable foundations" to support Russian poetry and philosophy and set up a union for Russian inmates.
During the 11-month trial, the court did drop one charge against Mr Khodorkovsky - relating to his 1994 acquisition of fertiliser company Apatit - as it fell outside the Russian justice system's 10-year statute of limitations.
His business partner Platon Lebedev was also jailed for nine years on the same charges; a third co-defendant, Andrei Krainov, was given a five-and-a-half year suspended sentence.
Mr Khodorkovsky and Mr Lebedev were also ordered to pay 17bn roubles (£330m; $600m) in taxes and penalties.
"I think [the sentence] is a testament to Basmanny justice," Mr Khodorkovsky told the court. Basmanny, the name of the court where the Yukos hearings began, has become a byword among his supporters for corrupt justice.
1963 - Born in Moscow; parents were chemical engineers
1980s - Sets up computer and software business with fellow students at Mendeleeva Chemical Technical Institute
1987 - Founds Menatep bank
1994 - Buys fertiliser company Apatit at auction
1995 - Buys oil company Yukos for $300m, with Menatep assuming $2bn in debt
October 2003 - Arrested on charges of embezzlement, tax evasion and fraud
June 2004 - Court case begins
May 2005 - Found guilty of six of seven charges and sentenced to nine years in jail
Commentators said that the sentence - just one year lower than the maximum that could be imposed - was a harsh one.
Outside the court, US Democrat congressman Tom Lantos said: "It seems that this political trial before a kangaroo court has come to a shameful conclusion.
"It is obvious that the conclusion of the trial was pre-determined politically and Mr Khodorkovsky could have been left at home and the trial could have been conducted without the prison circus."
Mr Khodorkovsky's supporters have claimed that the trial was politically motivated punishment from the Kremlin in response to his financing of pro-Western opposition political parties.
Mr Khodorkovsky's supporters have been demanding his freedom
"The sentence had to be long enough so Khodorkovsky was in jail in 2008, at the next presidential elections," political analyst Marsha Lipman at the Carnegie Moscow Centre added.
"There will be a lot of criticism in the West, no question about it."
However the Russian prosecutor's office "categorically denied" that the case was politically motivated.
"Specific serious crimes were committed and have been proved. Astronomical sums were stolen," spokeswoman Natalia Vishnyakova said.
The charges against him go back to Russia's hasty privatisation programme in the 1990s under which Mr Khodorkovsky was able to form Yukos, formerly Russia's largest oil company.
Since Mr Khodorkovsky has been behind bars, Russian authorities have broken up Yukos after hitting it with a bill for $28bn in back taxes.
Yukos, meanwhile, has sued the Russian government for 324bn roubles, arguing that it is due compensation for the break-up, which saw its main production unit sold off to a competitor.
The company says the break-up will eventually ruin it.