Venezuelan rights group Fundaredes said Friday authorities had arrested its director and two other activists who have lifted the lid on fighting near the border with Colombia.
Director Javier Tarazona and two others were taken by Venezuelan intelligence services, the NGO said on Twitter.
A fourth activist was arrested but released eight hours later, the group said, adding that the other three were transferred to Caracas.
Fundaredes had reported on the presence of Colombian dissident guerrillas on Venezuelan territory and criticized the response of the government, which it accused of harboring the fighters.
It reported on the fighting that broke out on March 21 before the government did. Clashes have since displaced thousands of civilians.
Venezuela does not name the armed groups it blames for the unrest, apart from calling them "terrorists" or linking them to drug trafficking or to Colombian President Ivan Duque.
However, security sources in Colombia say they are likely dissidents of the now-disbanded Revolutionary Armed Forces of Colombia (FARC) rebel group, an analysis Venezuela's Nicolas Maduro has conceded was possible.
Bogota has long accused Venezuela of shielding members of the FARC and armed rebel group ELN on its soil -- a charge Maduro denies.
Some FARC fighters who refused to join Colombia's peace process have continued their struggle, while also mixing with and battling drug traffickers.
Venezuela and Colombia, which share a 2,200-kilometer border, severed diplomatic ties in January 2019, after Bogota recognized opposition leader Juan Guaido as the leader of Venezuela over Maduro following a disputed election.