Robert Kocharyan was Armenia's president between 1998 and 2008.
The 71-year-old is now leader of Armenia Alliance, the Caucasus country's second-largest opposition party by number of seats in parliament, while his eldest son, Sedrak, is a businessman, AFP reported.
Armenia's Anti-Corruption Committee accused the pair of being involved in "criminal schemes", alleging Kocharyan used his position as president to purchase state property at "prices significantly lower than market prices".
Among them was a plot of land belonging to the defense ministry, which was sold to an associate of Kocharyan for quarter of the market price, the agency said.
Robert Kocharyan did not immediately respond to the allegations.
A message posted on his Telegram account described the arrests as an "attack by the authorities on President Kocharyan and his family".
Kocharyan was taken to hospital following his arrest due to high blood pressure, the anti-corruption agency later said.
The arrests come a day after Kocharyan's younger son Levon, an opposition lawmaker, criticized Prime Minister Nikol Pashinyan in parliament, suggesting he was to blame for the loss of "Artsakh" -- an Armenian-populated breakaway state in Azerbaijan that collapsed following an offensive by Azerbaijani forces in 2023.
Pashinyan responded by accusing Levon Kocharyan of obtaining his wealth "at the expense" of Armenia.
Pashinyan, who rose to power on the back of a 2018 street revolution, has often portrayed himself as a man of the people and supports closer ties with the West.
The 51-year-old's party was re-elected in June, although he has faced allegations of democratic backsliding from the opposition.
The leader of the largest opposition party -- Armenian-Russian billionaire Samvel Karapetyan -- was arrested last year on suspicion of calling to overthrow Pashinyan, charges he strongly rejected.