The United States government has warned of "serious consequences" after Venezuela's government moved to freeze the bank accounts of self-proclaimed interim president Juan Guaido.
It comes as Venezuelan state-run oil company PDVSA sought to sidestep US sanctions.
The sweeping sanctions on PDVSA, announced yesterday and aimed at curbing crude exports to the US and driving President Nicolas Maduro from power, were the strongest measures yet against the 56-year-old former union leader who has overseen economic collapse and an exodus of millions of Venezuelans in recent years.
The measures triggered higher global oil prices, angry responses from China and Russia and the first serious moves against Mr Guaido since he challenged Mr Maduro's claim on the presidency last week.
Venezuelan Attorney General Tarek Saab said he had asked the Supreme Court to open a preliminary investigation into Mr Guaido, accusing him of helping foreign countries to interfere in internal matters.
He also asked the court to impose a travel ban on the 35-year-old leader and to freeze his bank accounts.
In response, US national security adviser John Bolton warned of "serious consequences for those who attempt to subvert democracy and harm Guaido," in a tweet that described Mr Saab as the "illegitimate former Venezuelan Attorney General."
The United States and several other countries have recognised Mr Guaido as Venezuela's legitimate head of state and denounced Mr Maduro as a usurper.
Mr Maduro, sworn in on 10 January for a second term after disputed elections last year, accuses Mr Guaido of staging a US-directed coup against him.
Mr Maduro is backed by a number of countries, including Russia.
Mr Maduro's inauguration sparked protests throughout Venezuela. Over 40 people are believed to have been killed in political violence last week, including 26 shot by pro-government forces, five killed in house raids and 11 during looting, UN human rights spokesman Rupert Colville said.
He said more than 850 people were detained between 21 and 26 January, including 77 children, some as young as 12.
Mr Guaido said he did not underestimate the threat of imprisonment but did not believe it was "anything new."
Many opposition leaders have been imprisoned in the South American nation.
"We are here. We will keep acting and working to confront the humanitarian crisis," Mr Guaido told a news conference.
Most experts believe the sanctions and other measures against Mr Maduro will encourage him to step down only if he loses the support of the powerful military, which until now has been mostly loyal to the leftist ruling party founded by late President Hugo Chavez.
Mr Maduro appeared on a television broadcast from a military base, praising the soldiers' loyalty.
As a politician who heads the National Assembly, Mr Guaido has immunity from prosecution unless ordered by a top court.
However, the Supreme Court is loyal to Mr Maduro and is expected to quickly open the investigation.
The US State Department said it had certified Mr Guaido's authority to control certain assets held by the Federal Reserve Bank of New York or other US-insured banks, including government and central bank accounts.
PDVSA responded to the sanctions by ordering customers with tankers waiting to load crude destined for the United States to prepay, according to sources. Such prepayment could be in violation of the sanctions, setting the stage for a standoff at the ports.
PDVSA is also seeking to sidestep the sanctions by asking major buyers, including US refiners, to renegotiate contracts, sources said.
Russia has condemned the sanctions as illegal interference, while China said they would lead to suffering for which the US would bear responsibility. Both countries have lent billions of dollars to Venezuela and are concerned about new stress on debt payments.