Shortly before the ceremony, opposition leader and Miranda State Gov. Henrique Capriles told reporters in Caracas that the ceremony was "an abuse of power."
"To be president, the people have to elect you," he said. "The constitution is very clear."
Capriles said Maduro would have to first register as a candidate and then campaign. "That's what the constitution says," he said. "It's all here."
Then, addressing Maduro as if he were there, he said, "Nicolas, they didn't elect you. The people didn't vote for you, boy."
He continued, "We are not going to permit that the sorrow that the people feel be an excuse for the abuse of power, for constitutional fraud."
He called the ceremony "an illegitimate swearing-in" and blasted a high court decision declaring it legal.
"Gentlemen of the Supreme Court, you do not decide who is and who is not president," he said. "The people decide ... The vice president takes charge, but not as interim president."
Asked whether he planned to run for president, Capriles demurred. "We will talk again," he said.
The opposition was arguing that Maduro should have served both as interim president, without being sworn in, and vice president, said Jennifer McCoy, director of the Americas Program at the Carter Center and professor of political science at Georgia State University. "But now that he's named a vice president and has the presidential flag, we'll have to see what they do about participating in the election," she said.
"A constitution cannot specify every single scenario that could occur," she continued. "That's part of the problem."