The protesters eventually retreated once the clouds of gas had spread throughout the street in downtown Caracas.
"We are blocking access because in agreement with the permit, the march is only authorized to reach here, " Alirio Ramirez, a national guard commander, said.
Ivon Lamprea, a local health official, said that at least 15 people were injured, including a police officer and a protester, who was hit in the face by an object, as well as others who were treated for gas inhalation.
Not far from the march, thousands of public workers and pro-Chavez union members took part in a march organised by the government.
Antonio Ledezma, mayor of metropolitan Caracas, pointed out the disparity between the handling of the two groups of protesters.
"Why is one group allowed to march where they want, while other Venezuelans face barricades and fences," he asked.
Anti-government protesters carried posters of Manuel Rosales, the main opposition leader, who was granted asylum in Peru after he escaped corruption charges he claims are politically motivated.
Venezuela's government has recently stripped opposition mayors and governors of their powers, specifically their control over airports, hospitals and the Caracas police force.