"Yes, we are in combat, keep morale high in this fight to disarm any traitor, any coup plotter," Maduro said at a televised event with the military high command in which he appeared surrounded by soldiers.
That declaration came after a troubled 48 hours for Venezuelans. May Day clashes between opposition supporters and Venezuela's Armed Forces in Caracas left a woman dead and 46 people injured on Wednesday, after Guaidó attempted to rally demonstrators against Maduro.
Jurubith Rausseo, 27, died at a clinic after being hit by a "bullet in the head during [a] demonstration," the non-governmental Venezuelan Observatory of Social Conflict said on Twitter, condemning her "murder."
Human rights organisations and health services reported 46 people injured in Wednesday's clashes, including one person with a gunshot wound.
Tensions in Venezuela have soared since Guaidó, who heads the National Assembly legislature, invoked the Constitution to declare himself acting president on January 23, claiming Maduro's re-election last year was illegitimate.
National Guard troops fired tear gas at stone-throwing protesters attempting to block a highway close to the air base in eastern Caracas where Guaidó had tried on Tuesday to spark a military uprising.
A second day of confrontations between opposition supporters and Maduro's security services came as the United States said it was prepared to take military action, if necessary, to stem the crisis in the troubled nation.
At least one journalist was injured when National Guard soldiers fired rubber bullets at a group of reporters covering the clashes.
Miguel Ramírez, 17, told AFP at one medical centre that he had been shot in the foot while protesting on the highway near the La Carlota air base. "I didn't manage to run and hide," he said.
Guaidó rallied his supporters in Caracas in the Labor Day demonstrations, urging them to stay in the streets. His appeal came despite the apparent failure the day before of a revolt by some soldiers and members of the Bolivarian National Guard who joined his side.
In Tuesday's clashes, one person was killed and dozens injured, according to human rights monitors. More than 150 people were arrested, the government and human rights organisations said.
UN Secretary General António Guterres warned Venezuela's authorities not to use deadly force against demonstrators, while the US and Russia accused each other of making the crisis worse, evoking Cold War confrontations of the past.
In a phone call with Russian Foreign Minister Sergei Lavrov, US Secretary of State Mike Pompeo accused Moscow of "destabilising" Venezuela.
Lavrov, in turn, charged that US interference was "destructive" and "in flagrant violation of international law."
"There is nothing for workers to celebrate," Guaidó told supporters in the oil-rich country suffering from hyperinflation and food and medicine shortages that have driven millions to flee. "We're going to remain in the streets until we achieve freedom for the Venezuelan people.
"The regime will try to increase the repression. It will try to persecute me, to stage a coup d'état," said Guaidó, who has been recognised by more than 50 countries, including Argentina, as the country's interim president.
He said staggered industrial action would begin later today, leading to a general strike.
Before thousands of his own supporters in front of Miraflores palace on May Day, Maduro declared he will have "no hesitation" to lock up those responsible for this "criminal coup d'état," threats recalling those he made the previous day.
In his Wednesday address, Maduro alleged that the "so-called coup d'état" had been organised "from the White House" by US National Security Advisor John Bolton.
Hours after the revolt by military members appeared to have fizzled out, Pompeo told CNN he believed Maduro was ready to flee to ally Cuba before he was dissuaded by Russia – a claim Maduro later refuted as "a joke."
"We're living through hell," a resident of western Caracas, Evelinda Villalobos, 58, told AFP.
"I believe the people in the streets will be the straw that breaks the camel's back."
Another demonstrator, Patricia Requena, 40, said that "yesterday we saw soldiers recognizing our interim president. We have to stay in the streets."
She vowed: "I'll keep demonstrating as long as God allows me to."
Michael Shifter, an analyst with the Inter-American Dialogue, told AFP the US approach to Venezuela was "unhelpful and often counterproductive."
"The US is right to back Guaidó in his battle against Maduro," said Shifter. "But beyond being on the right side, the administration is making it harder, not easier, to achieve a democratic transition in Venezuela."