Abd al-Aziz al-Tayyar told Aljazeera by telephone that Saudi security forces had surrounded his home just after 1am (22.15 GMT) and were preparing to storm his house.
But in his last few minutes of freedom, the activist told millions of viewers how “all tribesmen are now willing to fight this government … we will protect the rights of our people some day.”
“This is not the Kingdom of Saudi Arabia any more. It is a jungle full of monsters … Saudi people are suppressed. They suffer poverty and unemployment,” he added.
Arrests followed as a number of activisits were arrested at al-Tayyar’s house in al-Rawda quarter of Riyadh, including Muhammad al-Ma’yuf, Khalid al-Turki and Muhammad al-Subai.
Al-Tayyar will be held at the Alaysha prison for questioning before being transferred to the notorious al-Hair political prison in Riyadh, according to Saudi dissident Dr Saad al-Faqih.
Al-Faqih believes the activist is unlikely to be tortured as he comes from a powerful and well-connected family, "but he may be made to sign a confession admitting wrong-doing".
Saudi authorities were not prepared to say why he was arrested.
Possible arrest reasons
Al-Faqih, now based in London, also said al-Tayyar has been arrested due to his numerous contributions on a pirate radio station in the UK.
The underground station, called al-Islah and carried by the Hotbird satellite on audio frequency 11096, has been operating for more than three months now.
Thousands of Saudis are understood to tune into the dissidents' station on a daily basis to listen to people like al-Tayyar.
The 34 year-old reformer formerly worked at the Saudi Chamber of Commerce, but was dismissed three years ago.
Over the last three months, however, he has become more vocal in his political opposition - even using his name on al-Islah Radio broadcasts.
Following his arrest, Faqih said that "dozens of contributors" to al-Islah were willing to give their names and speak up against injustice.
Hostages in Jizan
In a separate incident, four armed men broke into a housing unit of King Fahd hospital where they held a few foreigners hostage, one witness told Aljazeera on Tuesday.
"This is not the Kingdom of Saudi Arabia any more. It is a jungle full of monsters"
Abd al-Aziz al-Tayyar,
Abd Allah Abu Izz al-Din said he saw about 80 Saudi police exchange fire with the armed men, killing four.
“Special Forces used tear gas to free the hostages,” he added.
Two other gunmen were also seriously wounded and at least two Saudi policemen died in the skirmish to the northeast of the city. No names have been released by Saudi authorities.
The hostage-breakers broke into the compound after escaping a police operation to arrest them at a nearby farm on Tuesday morning.