Vladimir Putin has followed through on weeks of threats by signing a decree that foreign countries they must start paying for gas in Russian roubles or it will halt supplies.
And those new rules start on Friday, meaning Western nations were effectively handed a midnight deadline to comply.
"Nobody sells us anything for free, and we are not going to do charity either - that is, existing contracts will be stopped," the Russian president said.
Many payments for April gas deliveries are reportedly not due until later in the month, so it is not thought that there is an immediate threat to supplies. It is also still unclear whether the new payment mechanism set out by Russia would fully ban euros.
But France and Germany condemned Mr Putin's demands as being akin to "blackmail".
Western companies and governments have previously rejected Russia's demands to pay for gas in roubles as a breach of existing contracts, which are set in euros or US dollars.
Yet the EU gets about 40% of its gas and 30% of its oil from Russia, and there is no ready-to-go replacement. Notably, the EU did not enact sanctions on Russian fuel supplies - even though other Western nations such as the US and Canada did.
Mr Putin's demand to be paid in roubles is widely seen as an attempt to boost the currency, which has been hit by the wide range of international sanctions that followed the invasion of Ukraine.
