Venezuelan President Nicolas Maduro has announced a ban on US-backed self-declared interim leader Juan Guaido holding public office.
But National Assembly legislature speaker Mr Guaido immediately shot back that the 15-year prohibition announced on state television by Mr Maduro's Auditor General Elvis Amoroso was invalid.
"He is not auditor general.... The legitimate congress is the only one with power to designate an auditor general," he said.
The announcement also prompted a withering response from the US, where State Department spokesman Robert Palladino described the move in a brief remark as "ridiculous".
It was the latest scuffle between Mr Maduro and Mr Guaido, who lay rival claims to be the legitimate leader of the oil-producing South American nation of 30 million people.
The competition has been escalated into a geopolitical struggle, drawing in the US and its allies which support Mr Guaido, while Russia, Cuba and China back Mr Maduro.
Though unpopular in a country spiralling ever deeper into economic chaos, Mr Maduro has the upper hand at home, thanks to loyalty from his military chiefs and, since last weekend, the presence of 100 Russian troops.
Abroad, Mr Guaido is buoyed by US sanctions against Mr Maduro's regime funnelling funds his way, and efforts to have his envoys recognised over Mr Maduro's in diplomatic missions and international organisations.
Mr Maduro, who so far has heeded US warningsnot to arrest Mr Guaido under threat of unspecified repercussions, appears nonetheless more confident since Russia's overt protection.
Russia has shrugged off a demand made a day earlier by US President Donald Trump that "Russia has to get out" of Venezuela.
Its troops, described as military experts, will stay "for as long as needed," a spokesman for Russian President Vladimir Putin said, suggesting Washington back off and not interfere.
On Monday, Foreign Minister Sergei Lavrov accused the US of organising a "coup" in Venezuela.
Some reports said the soldiers were there to operate air defence systems bought from Russia.
The US has reacted angrily to Russia's move, with Secretary of State Mike Pompeo expressing Washington's displeasure in a tweet that borrowed a Maduro hashtag usually aimed at the US.
"Maduro calls for hands off Venezuela while he invites security forces from Cuba and Russia, so he and his cronies can keep plundering Venezuela," Mr Pompeo posted.
"It is time for Venezuelan institutions to stand for their sovereignty. Russia and Cuba, #HandsOffVenezuela."
The US tactics, so far, have been focused on ratcheting up sanctions. Those will jump to a critical level for Venezuela in a month's time, on 28 April, when the US will impose a ban on imports of Venezuelan oil.
With the United States as Venezuela's main customer for crude, that step is expected to severely impact Mr Maduro's already depleted state coffers.
The standoff has seen the US pull its diplomats from Venezuela, and American Airlines, which had already temporarily suspended flights to the country over security concerns, said yesterday that it was doing so indefinitely.
Months of increasingly tough US sanctions, coupled with mismanagement by the Maduro regime as it took over businesses and key sectors of the economy, and also corruption and many years of infrastructure under investment, have taken a toll.
Between Monday and late Wednesday, Venezuelans endured their second nationwide blackout this month. The previous one, unprecedented in scale and duration, lasted a week.
The outages knocked out transport, water supplies, communications and bank card terminals, closing schools and public offices, while also reducing frozen stocks of already scarce food.
Analysts said the oil sector, which brings in 96% of state revenue, was all but paralysed.
For ordinary Venezuelans, the blackouts were a devastating blow on top of the hyperinflation and food insecurity that were pushing many to the brink of survival.
"This makes anyone desperate... Enough!" said Mauro Hernandez, a 57-year-old Caracas resident forced to walk 90 minutes to get to work.
Mr Maduro's government blamed the blackouts on US "cybernetic" attacks and "terrorist acts" by the opposition.
Communications Minister Jorge Rodriguez has asserted that power had been restored to "most" of Venezuela and said that public offices - but not schools - will reopen today after a multi-day closure.
But many continue to leave the collapsing country, which has been abandoned by nearly three million Venezuelans since 2015, according to the UN.
An internal UN report obtained by AFP said that about 24% of the country's population - seven million people - are in dire need of humanitarian aid.