Venezuela is bracing for rival demonstrations — and the potential for worsening political violence. An opponent of President Nicolás Maduro joins a protest. (Federico Parra/AFP via Getty Images)

Anti-government protesters poured into the streets of Venezuela’s capital and blocked its main highway Wednesday, but their march toward downtown Caracas was stopped by volleys of tear gas fired by riot police and national guard troops.

The march was by far the largest in a wave of protests this month, as opponents of President Nicolás Maduro have sought to increase pressure on his unpopular government. Dramatic images spread by social media showed protesters fleeing clouds of gas by scrambling down embankments into the city’s badly polluted Guaire River.

In another video spread widely, a solitary protester stood in the path of an armored police vehicle, forcing it to back up, while canisters of tear gas fogged the streets around him.

Despite the huge crowds, Maduro remained defiant. Speaking at a counter-demonstration organized by the government, he dismissed the protests as part of a “coup” plot. Maduro threatened to hold opposition leaders accountable for the scattered outbreaks of violence on Venezuela’s streets.

(Sarah Parnass/The Washington Post)

“They have tried to topple us, and we’ve defeated them once more!” he told the cheering crowd of red-clad supporters, insisting he would not concede “one millimeter” to his opponents.

Wednesday’s march was a test for Maduro’s opponents, who have urged frustrated Venezuelans to join them despite their fears of out-of-control violence and a harsh crackdown by security ­forces. If their goal was to show that Venezuela’s masses have turned against the government, the large turnout suggested they were mostly successful in attracting people who have been on the sidelines.

“I don’t usually participate in protests, and today I choked on the tear gas,” said Teresa Ruiz, 58, an accountant who said she was trying to return home after a day in the streets. “It’s too much for an old lady like me.”

 At least two people were killed in scattered acts of violence that were also a sign of the dangers protesters face. One was a young man who died after being shot in the head by unidentified attackers at a Caracas intersection where marchers were gathering. The other victim was a 24-year-old woman shot at a protest in the western Venezuelan state of Táchira.

At least six others have been killed in this month’s unrest, according to U.S. officials — the worst political violence since clashes in 2014 that left more than 40 dead.

The oil-rich country’s economy has collapsed, leading to chronic shortages of food and medicine. Maduro’s critics are calling for new elections, the release of political prisoners and an end to autocratic abuses of Venezuelan institutions. Many chanted “Freedom!” as they marched Wednesday.

At an opposition gathering in western Caracas, Jose Rafael Rojas, 60, said he had taken a day off from work to join the march. “In Venezuela, there is no peace, no justice,” he said. “Having to search for food is very frustrating for me. Every time I look for corn flour, I can never find it.”

Citing the need to maintain order, the government took extraordinary measures to try to limit the size of Wednesday’s opposition march, cutting subway service and setting up roadblocks along highways.

The government also urged its supporters to take to the streets in Maduro’s defense. At least some employees at state-owned companies reportedly were threatened with dismissal if they declined to join pro-Maduro rallies.

In his speech Wednesday, Maduro repeated his claim that the United States was plotting to overthrow him and said security forces had captured more than 30 masked putschists he described as “terrorists.” He threatened to arrest opposition leader Julio Borges, the leader of Venezuela’s legislature, calling him a “coward,” a “clown” and a coup-monger.

Maduro cited a statement issued by the State Department on Tuesday titled “Non-Violence and Accountability During Protests in Venezuela,” calling it a “green light” for the conspiracy against him to go forward.

The statement calls for both sides to remain peaceful and said the United States joins 11 other nations in the region in urging Maduro’s government to take immediate steps toward “a prompt solution to the grave crisis affecting all Venezuelans.”

On Wednesday, Secretary of State Rex Tillerson said that “we are concerned that the government of Maduro is violating its own constitution and is not allowing the opposition to . . . organize in a way that expresses the views of the Venezuelan people.”

Maduro opponents were left seething after Venezuela’s supreme court tried to strip the ­opposition-controlled legislature of its powers last month, a decision judges mostly reversed the following day.

The government was also widely condemned for disqualifying opposition leader Henrique Capriles from running for office for 15 years, meaning that he will be ineligible to challenge Maduro for the presidency in elections due to be held next year.

Opposition leaders called for the protests to continue Thursday.