Colombia's President Juan Manuel Santos expressed "serious concern" about the Venezuelan army's role, after his and 10 other countries urged peaceful demonstrations.
Venezuela's President Nicolas Maduro rallied the military and civilian militia on Monday as he vowed to resist the opposition's efforts to remove him from office.
"We view with serious concern the militarization of Venezuelan society. We call for good sense," Santos wrote on Twitter.
The center-right opposition is protesting against moves by authorities to tighten Maduro's grip on power.
Pressure on Maduro has risen in an economic crisis aggravated by a fall in prices for Venezuela's crucial oil exports, which has triggered severe shortages of food and medicine.
Late on Monday, 11 regional powers including Argentina, Brazil and Mexico bewailed the deaths of several protesters in recent clashes.
They urged Venezuelan authorities to "guarantee the right to peaceful protest as enshrined in the constitution and to prevent any violent action against the demonstrators."
At least five people have been killed and scores injured in clashes between protesters and police this month, according to Venezuelan authorities.
Venezuela's defense minister on Monday declared the army's loyalty to Maduro, who has ordered troops into the streets ahead of Wednesday's protests.
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