(CNN) -- Vice President Nicolas Maduro was sworn in Friday as acting president of Venezuela in a ceremony held in the National Assembly in Caracas, three days after the death of President Hugo Chavez.
Asked if he swore to uphold the laws of the country and to follow the constitution, Maduro said, "I swear."
Then, in a rambling speech that evoked the bombastic oratory of Chavez, Maduro said he would work to keep alive his memory and legacy.
"We still have him in our hearts," said Maduro, who was wearing a sash bearing the colors of the Venezuelan flag across his chest. "I have him here, here, as if he was the name in my soul, because I am his son."
He continued, "We are here to guarantee peace, safety and political stability and the lifting up of the poor in Venezuela will continue. Onward and upward with socialism!"
Maduro appealed to the opposition to field a presidential candidate for elections that are to be held in 30 days. "Some of them are here today," he said. "Welcome."
But he noted that few opposition members were in attendance. "They didn't want to come," he said.
About the elections, he said, "May the best person win. The people will decide."
Maduro alluded to the United States, with whom Chavez had had prickly relations. "Our eyes will see sooner rather than later the decline of the imperialist elites that changed the United States into an empire of aggression," he said. "That moment will be a fabulous moment for humanity."
He announced that he had appointed the minister of science and technology, Jorge Arreaza, to replace him as vice president.
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."
She predicted that Chavez's primary legacy would be the sense of inclusion that he gave to those sectors of the population that had not felt represented in the past. "I think it's a very strong legacy and that they do feel like now they've got rights and that they will exercise those rights in the future."
But he also leaves a divided country, a move that was intentional, she said. "He felt that change could only be accomplished through confrontation," she said. "And that confrontation included excluding those who used to be in power."
The result has been a loss of much of the country's production capacity, both in the private sector and in the petroleum sector, where a number of workers and managers were fired after a strike in 2003, she said.
Michael Shifter, president of the Inter-American Dialogue, a Washington-based policy forum on Western Hemisphere affairs, predicted that a Maduro administration would find it tough to govern.
"Chavez had a unique ability to do that, and nobody else has either the political skill or the charisma to keep all the very disparate factions of chavismo together."
Still, Maduro and his supporters have great incentive to rise to the occasion. "If they don't, they risk losing power, and they want to stay in power," he said.
But even if he wins a six-year term as president, Maduro would not be on solid ground, given the country's profound economic and security problems, Shifter said.
He cited inflation above 22%, massive debt, shortages of basic goods, routine electricity blackouts and widespread crime -- the 16,000 people killed in an average year is triple the number who were killed the year Chavez entered office.
"The fundamental problem is that the government has just spent much more money than they had."
Shifter also predicted that a Maduro administration would not maintain warm relations with the United States. Neither country has posted an ambassador in the other country since 2008. "Maduro has to be very careful about the base of his party and following Chavez's policy toward the United States, which has been one of a lot of confrontation and aggression."
Still, he noted, Maduro is a former union official, not the paratrooper military officer that Chavez was. "So, he's somebody who has negotiated deals. I think he'd be open to at least having communication with the United States."
CNN's Esprit Smith contributed to this report.