Pro-government protesters threw stones Thursday at a bus that carried a delegation of Brazilian senators who traveled to Venezuela in a bid to visit imprisoned opposition leader Leopoldo Lopez, the politicians said. The senators were reportedly forced to abandon the visit and return to an airport in Caracas to await a return flight to Brazil.

The protesters besieged the Brazilian senators while they were en route to Ramo Verde military jail, where Lopez has been held since 2014, Reuters reports. At least two of the Brazilian senators took to Twitter to document the attack as it occurred.

“Our bus was under siege; they were beating and trying to break it. I filmed them throwing stones against the bus,” Brazilian Sen. Ronaldo Caiado wrote, according to the BBC.

A second Brazilian senator, Aecio Neves, echoed Caiado’s comments. “We are here to defend democracy, and until now the Venezuelan government has shown little appreciation of it,” Neves wrote on Twitter.

Brazil’s Senate and its Foreign Ministry both condemned the assault on the senators’ bus and said they would seek an explanation of the events from the Venezuelan government. Venezuelan President Nicolas Maduro’s government has yet to publicly address the situation. Maria Corina Machado, a prominent opposition leader, accused Maduro’s regime of purposely impeding the Brazilian senators’ visit.

“The Brazilian senators now know what it’s like to live in the dictatorship of today in Venezuela,” she told Reuters.

Lopez, 44, was imprisoned last year on the government’s accusations that he incited violent protests in which dozens of people were killed. Last month, Lopez said he would go on a hunger strike to demonstrate his objection to Venezuela’s treatment of political dissenters, the Guardian reported.

U.S. President Barack Obama issued an executive order in March which formally identified Venezuela as a national security threat and enacted sanctions against seven officials for their treatment of political dissidents and apparent human rights abuses.

"Venezuelan officials past and present who violate the human rights of Venezuelan citizens and engage in acts of public corruption will not be welcome here, and we now have the tools to block their assets and their use of U.S. financial systems," White House spokesman Josh Earnest said in a statement, according to Reuters.