Last week, the special additional sessions court in Kozhikode sentenced 11 of the 12 accused in the case to life imprisonment and also observed that the murder was the result of political rivalry not personal grudge.
Chandrasekheran, 51, who launched the Revolutionary Marxist Party after his exit from the CPI-M, was hacked 51 times by assailants May 4, 2012 when he was returning home on his motorcycle.
A hugely popular leader in and around the Kozhikode district, Chandrasekheran took on his former party, the Communist Party of India-Marxist (CPI-M), from which he was ousted in 2008, and criticised their ideological stand. CPI-M state secretary Pinarayi Vijayan publicly declared Chandrasekheran a "renegade".
"I will not relent till a CBI probe is announced into the case," Rema said.
She arrived in the state capital Sunday night to a rousing welcome from supporters at the railway station.
A huge number of people had gathered in front of the state secretariat Monday morning, awaiting her arrival to begin her indefinite fast.
State Forest Minister Thiruvanchoor Radhakrishnan arrived to meet Rema at the place where she began her fast.
Chandrasekheran was hacked 51 times by assailants in 2012.
In 2012 leader of opposition V.S. Achuthanandan shocked his CPI-M colleagues when, on the day of polling during a crucial by-election, he visited Rema's home to offer her moral support after her husband was killed.
"There is nothing to prevent a CBI probe. Those who are against it should come out in the open to state why it should not happen. All the state government should do is bring this before the cabinet and hand the matter over to the CBI," union Minister of State for Home Mulapally Ramachandran told reporters in Kozhikode Monday.
In the state assembly, former speaker of the house and CPI-M legislator K. Radhakrishnan demanded that police officials who beat up the nine accused in the case when they were brought to Viyur Central jail last week should be suspended.
"Nine accused were brought to the jail last Thursday. First they created a scene when they refused to allow jail staff to check their bags. Then they said that three of them should each be put in one cell. During a visit by Radhakrishnan to meet the accused, they said they wanted a medical examination. All of them were taken to the Thrissur Medical College the next day and then back to jail. A separate case has been registered against the nine accused in the case for misbehaviour in jail premises," state Home Minister Ramesh Chennithala said.