Speaking to Aljazeera on Tuesday, Abd al-Rahman al-Lahim, who is based in Saudi, had said the arrest of the reformers was "contrary to the law".
He advocated "freedom of expression, a priority for economic reform" envisaged by the authorities.
Sources close to the reformers on Wednesday said Saudi security forces had arrested some 10 reformers since Tuesday, including academics who were among 116 signatories to a petition to the government in December calling for transforming the kingdom into a constitutional monarchy.
The sources added that three of the reformers had been released on Wednesday while the rest were still in detention.
They named the three as Khalid al-Hamid and Adnan Al-Shakhs, both academics, as well as Abd al-Rab Abu Khamsin, an activist.
An official with the Saudi interior minister confirmed the arrest "of a limited number of individuals for questioning about statements they issued that do not serve national unity and the social fabric built on the rules of Islam," said the official SPA news agency.
The official provided no further details of the number or identity of those arrested.