Sen suffered from chest pain, fluctuating blood pressure and uneasiness.
According to the hospital sources, the minister is in the intensive critical care unit and examinations are being conducted on him.
Medical superintendent Debasish Sarma told PTI that a medical board, comprising seven consultant specialists, has been formed for Sen's treatment.
Sen was under observation. He had light breakfast in the morning, Sarma added.
Meanwhile, party sources said Sen had undergone angioplasty at New Delhi's All India Institute of Medical Sciences in 2003.
They said Sen had visited Jyoti Basu at his residence along with the other central committee members before he fell ill.