Venezuela's opposition leader Juan Guaido has declared himself interim president, winning backing from the United States, Canada and some South American countries.
US President Donald Trump formally recognised Guaido shortly after his announcement and praised his plan to hold elections.
US Secretary of State Mike Pompeo called on the Venezuelan military to protect "the welfare and well-being of all Venezuelan citizens," and said Washington would take "appropriate actions" against anyone who endangered the safety of US personnel.
The United States would conduct its diplomatic relations with Venezuela through "the government of interim President Guaido," he said.
But Russia, which has invested heavily in Venezuela's oil industry, provided support to its military, and become a lender of last resort as the economy implodes, denounced the declarations.
Russia's lower parliamentary house speaker Vyacheslav Volodin called the steps to remove Maduro illegal.
And a second lawmaker, Franz Klinzevich, said Moscow could wind up its military cooperation with Venezuela if Maduro was ousted.
Elsewhere, Turkish Foreign Minister Mevlut Cavusoglu said recognising a leader other than Maduro might cause "chaos".
In Paris, a presidency official said France was consulting with its European partners about the situation.
At a rally on Wednesday in the east of the capital Caracas that drew hundreds of thousands of Venezuelans, Guaido accused Maduro of usurping power.
He promised to create a transitional government that would help the oil-rich nation escape its hyperinflationary economic collapse.
"I swear to assume all the powers of the presidency to secure an end to the usurpation," 35-year old Guaido, the head of the opposition-run congress, told an exuberant crowd.
Guaido's declaration takes Venezuela into uncharted territory, with the possibility of the opposition now running a parallel government recognised abroad as legitimate but without control over state functions.
In a televised broadcast from the presidential palace, Maduro accused the opposition of seeking to stage a coup with Washington's support.
"We've had enough interventionism, here we have dignity, damn it! Here is a people willing to defend this land," said Maduro, flanked by Socialist Party leaders.
Defense Minister Vladimir Padrino said on Wednesday that the armed forces did not recognise a self-proclaimed president "imposed by shadowy interests ... outside the law."
Military top brass indicated their continued support for Maduro in tweets.
Australia has also expressed worry about the deep political unrest.
"We are very concerned at what is a clearly deteriorating political, economic, security and humanitarian situation in Venezuela," Foreign Minister Marise Payne told reporters in Sydney on Thursday.