Muhammad al-Maghrabi, a 64-year-old anti-corruption activist arrested on charges of practising law without a licence, was due to be released on a bail of $265.
But prosecution lawyers have been given 24 hours to appeal the bail release, according to a fellow activist campaigning for al-Maghrabi's release, Kamal al-Batal.
"What is happening is surprise after surprise," he added. "For me nothing has changed. Until he is out, he is not out."
Al-Maghrabi, who founded a campaign for integrity in the judiciary, was arrested on 8 August after Lebanon's bar association accused him of illegally practising law.
Rights groups have called the arrest politically motivated. But Lebanon's public prosecutor has denied the accusation, saying all the accused rights were protected.
If convicted, the rights lawyer faces anything from six months to three years in jail.
Working on politically sensitive cases in Lebanon, al-Maghrabi previously filed multiple complaints against the bar association, according to a website dedicated to his defence.
Human Rights Watch, which has called al-Maghrabi's detention "outrageous" and has demanded his release, said the Lebanese bar association had repeatedly tried to disbar Mugraby, but that he had continued to register as a lawyer.
He also made formal complaints about two Lebanese nationals who disappeared in 1997, were transferred into Syrian custody and then held incommunicado in Damascus.
Damascus poured troops into Lebanon early on during its 1975-1990 civil war and has maintained significant power and influence in the country ever since.