Lilian Tintori posted images on social media showing a document signed by immigration officials ordering the seizure of her passport as she was preparing to board a flight.
"They have just banned me from leaving the country," she tweeted. "What the dictatorship wants is to prevent us from doing a very important international tour."
Tintori said she was going on a trip to Europe to meet with French President Emmanuel Macron, German Chancellor Angela Merkel, United Kingdom Prime Minister Theresa May and Spanish Prime Minister Mariano Rajoy to discuss the crises in her country.
She also posted a picture from the airport standing with the ambassadors of Spain, Italy and Germany, who she said became "witnesses to this outrage by the dictatorship."
Speaking at a news conference after the incident, Tintori said the trip to Europe cannot be stopped.
"This cannot be stopped, this tour will happen," she said. "What is happening in Venezuela is unstoppable. They can't silence our voice."
"The prohibition to leave the county won't silence the voice Venezuela, won't silence the voice of the Venezuelans, won't silence my voice," Tintori said.
There was no immediate statement on Tintori's remarks or situation from the Venezuelan government.
European leaders condemned the actions of the Venezuelan government.
Spanish Prime Minister Mariano Rajoy called the move "regrettable." "The prohibition of @liliantintori to leave is regrettable. They can lock people but not ideals. Freedom for Venezuela. MR," he tweeted.
"We are waiting for Lilian Tintori in Europe," French President Emmanuel Macron said on his official Twitter feed. "The Venezuelan opposition must remain free."
Venezuela's Foreign Minister Jorge Arreaza said he is "rejecting the behavior" of the French President, who has been highly critical of the Venezuelan government.
"We demand the French Government to respect the functioning of our institutions," Arreaza tweeted. "The French president ends up supporting those who have committed grave and notorious crimes of corruption, which in his country are severely punished."
Venezuela has been torn by strife for months. Anti-regime protests opposing the government of President Nicolas Maduro have turned deadly and a looming economic crisis have left the population in dire need for food and medicine.
In March the Supreme Court attempted to dissolve the National Assembly, leading to the latest wave of unrest.