Suarez, Spain's first prime minister after the death of General Francisco Franco, has suffered from Alzheimer's for most of the past decade.
"The disease has progressed a lot and everything indicates that the end is imminent," his tearful son, Adolfo Suarez Illana, told a hurriedly organised news conference at the Cemtro hospital where his father was admitted on .
"The time horizon we are looking at is 48 hours but we are in God's hands. We are just going to help him medically so he has no suffering and let him leave in peace," he said.
"It is imminent and it could be much faster than we think," he said.
Last rites had already been administered to the dying Suarez, the son added.
The former Spanish leader's son said he had spoken to King Juan Carlos and Prime Minister Mariano Rajoy.
The effects of Alzheimer's meant that the former prime minister did not enjoy an intellectual relationship with his family but he did have an "emotional relationship", the son said.
"These past two days have been happy," he added. "He has given us more smiles perhaps than in the past five years."
Suarez is one of the last surviving players in Spain's historic "transition" -- the delicate dismantling of dictatorship followed by democratic reforms that he and King Juan Carlos helped achieve after Franco died in 1975.
"His role in the transition was second only to that of the king," said the historian Javier Tusell.
Despite being born the son of a Republican, Suarez became a member of Franco's regime, serving as head of the state broadcaster and a senior leader in the National Movement, a Francoist party with fascist roots.
The king, Franco's successor as head of state, named Suarez prime minister in a new government in 1976 at the age of 44, and he was confirmed as leader in an election the following year.