BOGOTÁ, Colombia — Members of President Nicolás Maduro’s governing party marched triumphantly into Venezuela’s Capitol building on Friday, calling to order a 545-member body with plans to rewrite the Constitution and consolidate their power over the nation.
The constituent assembly, as the group is called, took a symbolic jab at their political rivals, parading through the gates of the legislative chamber holding portraits of former President Hugo Chávez, which were taken down just last year after opposition parties won control of the National Assembly.
“This assembly didn’t emerge from nothing,” said Delcy Rodríguez, a former foreign minister close to Mr. Maduro who will lead the body. “It has dodged the obstacles thrown in its way by those who resist democracy.”
The convening of the assembly was the culmination of an ambitious plan by the president to secure political control over Venezuela. In a contentious election on Sunday, Mr. Maduro instructed Venezuelans to choose delegates from a list of allies in the governing party. Voters were not given the option of rejecting the plan.
The body will rule Venezuela for however long it takes its members to rewrite the Constitution, giving virtually unlimited power to Mr. Maduro’s party.
Among the new leaders were Mr. Maduro’s wife, Cilia Flores; the president’s son, Nicolás; Diosdado Cabello, a powerful former military chief who participated in a coup in the 1990s with Mr. Chávez; and a radical television show host known for broadcasting embarrassing recordings of opposition politicians.
Before the ceremony, crowds of government supporters surrounded the Capitol, many dressed in red, the color of Mr. Maduro’s Socialist party, waving flags and dancing to salsa music on speakers. The festive atmosphere was in marked contrast to the months of antigovernment protests and clashes that have left more than 120 people dead and to the hardships faced by Venezuelans dealing with shortages of food and medicine.
On Friday, Mr. Maduro urged assembly members to move swiftly to resolve these problems.
“The mandate of this constituent assembly is to use its powers to make peace, to construct peace, for a new economic model,” he told supporters.
Mr. Maduro’s own authority under the assembly — which is technically above the president — was a cause of speculation among some analysts in recent days, who noted that the president’s rivals within his party might try to take control of the assembly and sideline Mr. Maduro.
But the choice of Ms. Rodríguez, a trusted deputy of the president, to lead the assembly signaled that Mr. Maduro aimed to maintain a firm grip over the assembly’s decisions. Another pick for a leadership post was Aristóbulo Istúriz, a former vice president who had been involved in past talks with the opposition, suggesting that the party’s more radical figures were being kept from the top jobs.
The fate of the National Assembly, the opposition-controlled legislature, was unclear on Friday. Many members of the constituent assembly have called for the legislature to be dismantled, or even for its lawmakers to be jailed. Several countries sent their ambassadors to join the Venezuelan legislators’ sessions, fearful that their chamber would be overrun by force.
But the assembly on Friday chose not to gather in the National Assembly’s chamber, but rather in an adjoining hall in the Capitol, leaving open the possibility that the dueling government branches may — at least for now — coexist.
Elsewhere in Caracas, the capital, a number of supporters of the opposition marched to protest against the constituent assembly and were met by government forces who beat them back. At least one lawmaker was injured, according to the opposition. But the number of protesters was far smaller than those who had turned out for previous marches.
The opposition received some good news on Friday when the Venezuelan authorities released Antonio Ledezma, an opposition leader, from custody and placed him under house arrest, according to his wife, Mitzy Capriles de Ledezma. Mr. Ledezma was taken to a military jail as part of a crackdown this week against opposition politicians.
Politicians vowed to continue their fight in the streets on Friday. “Today the Venezuelan people have demonstrated that they have the power,” said María Corina Machado, another opposition leader, in a video from the march. “The power is here, the power is not in this Dante-esque circle of hell that they’re installing” in the Capitol building.
On Friday, the Vatican issued a statement urging the government to do away with the constituent assembly, saying the body “instead of fostering reconciliation and peace” would “foment a climate of tension.” It joined more than 20 countries that have objected to the assembly, including the United States, which this week imposed sanctions on Mr. Maduro and called him a dictator.