The narrow victory by Maduro in Sunday's presidential vote has been rejected by his rival, Henrique Capriles, who is alleging thousands of irregularities at polling centers and wants a full audit of the ballots.
Eight people have died in opposition-led protests and the government has vowed legal action against Capriles and others whom they accuse of stirring up violence against its backers.
Washington said it had not decided whether to recognize Maduro, a former bus driver-turned-foreign minister who was picked as successor by the late socialist leader Hugo Chavez.
"We think there ought to be a recount," Secretary of State John Kerry told U.S. lawmakers. "Obviously, if there are huge irregularities, we are going to have serious questions about the viability of that government ... I'm not sure that's over yet."
Maduro responded by accusing Washington of directly supporting the Venezuelan right-wing "like never before" in a war against the people and Chavez's revolution.
"The U.S. intervention in Venezuelan internal affairs in recent months, and particularly during the election campaign, has been brutal, vulgar," he said in a televised speech.
"Its direct coordination with the 'yellow bourgeois', with the oligarchs, has been truly obscene ... Take your eyes off Venezuela, John Kerry! Get out of here!"
During Chavez's 14-year tenure, Venezuela was the U.S. government's main irritant in Latin America and the former soldier frequently invoked "imperialist" plots against him.
The latest instability in the OPEC nation with the world's largest oil reserves has sent Venezuelan bond prices tumbling.
The unrest, just weeks after Chavez's death from cancer, has laid bare the deep polarization of a country split down the middle between pro- and anti-government factions, and left its 29 million people on edge.
Several South American presidents will hold an emergency meeting on Thursday in Peru to discuss the situation. It was not clear if it would produce a show of support for Maduro, or whether the leaders wanted to call for calm.
Capriles has accused the government of ordering gangs to attack his supporters and even his official residence in Miranda state, where he is the governor.
Dozens of government supporters gathered peacefully outside the building on Wednesday, chanting "Capriles is a murderer!" and "Fascist!" while being watched by troops.
Though demanding legal action against his opposition rival, Maduro nevertheless said he would be protected by the state.
OPPOSITION MARCH OFF
Capriles had planned to lead a march on the National Electoral Council (CNE) headquarters in Caracas on Wednesday, but Maduro banned it. Capriles later called it off, saying the government had plotted to start trouble and then blame him.
"Whoever is involved in violence is not part of this project, is not with me," he said. "It is doing me harm."
Instead of the rally, opposition officials filed papers with the CNE formally requesting a recount.
Monday's scenes of opposition supporters attacking ruling Socialist Party offices, government-run clinics and people celebrating Maduro's victory were damaging to Capriles' cause, which he casts as one of democracy versus autocracy.
Both sides have accused the other of supplying gangs of armed thugs with shirts in each other's party colors and orders to attack people and destroy property - seeking to ensure that their political rivals got the blame.
Evoking the emotive memory of a 2002 putsch against Chavez, which lasted only 48 hours but led to a radicalization of the government and a discrediting of Venezuela's opposition, Maduro has accused Capriles' camp of planning another coup d'etat.
About 135 people were arrested and more than 60 hurt during Monday's clashes, officials said. Tuesday's protests were much quieter, with thousands of Capriles' followers holding peaceful rallies outside CNE offices around the country.
The most successful and charismatic leader the opposition has had since Chavez took office in 1999, the 40-year-old Capriles says the government is responsible for the violence because it denied reasonable requests for a full recount.
Maduro won with 50.8 percent of the vote against Capriles' 49.0 percent, according to the electoral authority. He is due to be formally sworn in on Friday.
Maduro campaigned on a pledge to continue Chavez's policies. He had a big lead in polls but that evaporated in the final days and the result was much closer than his team expected.
They have defended the legitimacy of his narrow win with repeated references to the 2000 U.S. election dispute, when the U.S. Supreme Court halted a recount in Florida and George W. Bush was declared the winner in the state by just 537 votes.
Capriles says he is sure he won and that his team has evidence of 3,200 irregularities, from voters using fake IDs to intimidation of volunteers at polling centers. Opposition sources say their count showed Capriles had an extra 300,000 to 400,000 votes not shown in the official tally.
The CNE has refused to hold a recount, saying an audit of ballots from 54 percent of the polling centers, in a widely respected electronic voting system, had already been done.
Maduro initially said he was open to a recount but has changed his position. He has called on his supporters to demonstrate all week, culminating in a big rally in Caracas on Friday to coincide with his inauguration ceremony.
Maduro's narrow victory has raised doubts about whether the disparate alliance that formed around Chavez can hold together without him. The opposition is also a wide-ranging coalition of parties from right to left on the political spectrum.