* Rouble falls to 74.95 vs dollar, 85.74 vs euro
* Oil prices plummet after no deal with OPEC
* Russian companies' GDRs post double-digit losses in London
* Russian cenbank halts FX buying for reserves
* Falling rouble dents household incomes, business activity (Adds stocks, updates prices)
MOSCOW, March 9 (Reuters) - Russia's rouble crashed to its weakest level since early 2016 on Monday as Moscow and OPEC failed to reach an oil output deal, prompting Saudi Arabia to slash its crude selling prices and sending oil prices tumbling.
The rouble slumped to 74 versus the dollar on the interbank market as of 0914 GMT after falling to 74.95, a level last seen in late February 2016.
Against the euro, the rouble was at 84.17, having briefly hit 85.74, its weakest since February 2016.
The rouble ended on the Moscow Exchange on Friday at 68.57 against the dollar and 77.51 versus the euro .
Shares in Russian companies also fell in London as the Moscow Stock Exchange was closed for a long weekend.
Global depositary receipts (GDRs) in oil giant Rosneft were down 20.4%, GDRs of Russia's No.2 oil producer Lukoil fell 18.5% and gas producer Novatek saw its GDRs weaken by 16.7%
Yet another drop in the rouble and oil prices could be sensitive for authorities and President Vladimir Putin, who last week blamed low oil prices on falling household incomes.
The falling rouble usually fuels concerns about inflation as rises in the dollar and euro filter into prices in Russia as imports become more expensive.
Rapid moves in the rouble also suffocate business and investment activity as companies tend to play forex markets instead of focusing on their business.
Russian authorities were quick to respond to the rouble's fall, with the central bank suspending its daily purchases of foreign currency for state reserves for 30 days, in an attempt to ease downside pressure on the rouble.
"The Bank of Russia is monitoring the situation on the financial market and is ready to engage additional instruments in order to preserve the financial stability," it said.
Oil prices have been under pressure in the past few weeks and extended losses after Russia balked at making a further steep output cut proposed by OPEC to stabilize oil markets hit by worry over the economic impact of the coronavirus.
Brent crude futures were down 20% at $36.18 a barrel after earlier dropping to $31.02, their lowest since Feb. 12, 2016.
Brent futures are on track for their biggest daily decline since Jan. 17, 1991, at the start of the first Gulf War.
Fitch rating agency said the drop in oil prices towards $30 per barrel poses risks of the rouble devaluation and budget deficit, the RIA state news agency reported.
Russian authorities said last week, before the latest sell-off in oil prices and the rouble, that Russia was fiscally prepared to cope with a drop in oil prices, pointing at Russia's international reserves of $570 billion.
For Russian equities guide see
For Russian treasury bonds see (Reporting by Andrey Ostroukh; Editing by Alex Richardson and Alexander Smith)