Maduro, a staunch ally of the Bolivian leader who resigned Sunday after weeks of protests, said the situation in Bolivia "could lead to a civil war".

"I call on the military high command of Bolivia: Be aware of what you are doing," Maduro told a meeting in support of Morales in Caracas.

"Your commander in chief, by constitutional order and vote of the people, is called Evo Morales Ayma and you must return him to power," he said.

"Don't repress the people. Do not kill the people."

The Venezuelan socialist leader has faced surges of popular unrest in his own country since 2014, which have left more than 200 people dead, amid a crippling economic crisis that has forced millions to flee.

Maduro said he had spoken with Morales by phone Tuesday following his arrival in Mexico.

"As soon as I could, I called him on the phone. He told me: 'Maduro, say hello to the people of Venezuela and tell them that I am strong and that I will return to Bolivia and there will be millions of us.' He was in good form."

But Maduro said Morales had been forced to resign "with a gun to his head, threatened with death" after the security forces withdrew their support after weeks of anti-government protests.