(Adds c.bank interventions)
By Yelena Fabrichnaya and Andrei Ostroukh
MOSCOW, Dec 26 (Reuters) - Russia's central bank allowed this month's eighth mini devaluation of the rouble on Friday, a day after the country's main export earner, oil, neared $30 per barrel, the lowest level since 2004.
The collapse of oil and other commodity prices coupled with the global economic slowdown and capital flight from emerging markets has turned the rouble around to a depreciation bet from an appreciation play in less than six months.
The central bank has spent more than $100 billion defending the currency in the last 4-1/2 months to prevent a run on banks and panic among the population, which still has sour memories of savings losses in the 1990s and Soviet times.
Dealers estimate the bank has spent around $10 billion supporting the currency in this week alone.
Faced with an economy potentially heading for its first recession in a decade, Russia started on a gradual depreciation path six weeks ago to preserve reserves -- still the world's third largest at $451 billion -- which will be needed to support the economy.
Russian authorities, which initially denied they would allow the rouble to fall sharply, have softened the tone in the past weeks, saying the rouble cannot remain strong during a sharp downturn on global commodities markets.
Russia had plans to balance its budget at an oil price of $70 per barrel next year but plans to revise it now to $50.
By 1500 GMT the rouble was trading at 34.31 versus a euro-dollar basket, having broken through the 33.86 mark seen as the previous central bank's support level.
A source at the central bank confirmed that the rouble's trading corridor had been widened again, but gave no details.
First Deputy Chairman of the central bank Alexei Ulyukayev, seen as Russia's key forex policy maker, said he will not alter his policy of allowing the rouble to weaken gradually, ruling out a one-off devaluation of the currency.
"This policy, from our point of view, is the least painful for market participants and firms. We think there is no reason to change it in favour of sharp jumps and fluctuations," Ulyukayev told Vesti 24 news channel.
The rouble has weakened versus the euro-dollar basket by almost 13 percent since the bank started its mild rouble devaluation in September.
The rouble eased beyond 29 per dollar on Friday, the weakest level in four years, and was traded at around 41 per euro, the weakest level ever.
The Economy Ministry sees the rouble rate at around 31-32 per dollar next year, but central bank executives say they dislike the forecast and the rate will depend on the euro/dollar rate as well as the oil price.
Traders said they expected another weakening before the end of the year as the central bank was seeking to take advantage of the fact that key foreign players betting against it were on holiday.
"Everyone, including the population, expects the rouble to weaken further. So I don't think there much reason here for the central bank to delay (another move)," said Nikolai Podguzov, an analyst at Renaissance Capital brokerage.
"The faster they do it... the better it is from the point of view of reserves' spending," Podguzov said, adding that Monday would be a good day to do it.
Standard & Poor's ratings agency downgraded Russia this month, the first time since the 1998 financial crisis, citing concerns over shrinking reserves.
On Thursday, Russia's mainstay Urals' crude oil export blend fell close to $30 per barrel but bounced back to $33.5-$33.7 on Friday together with global prices. (Reporting by Yelena Fabrichnaya and Andrei Ostroukh; Writing by Tanya Mosolova and Gleb Bryanski; Editing by Ruth Pitchford)