Vijayan said the "attack" by sangh parivar on the CPI-M leader at party headquarters is "shocking to democratic consciousness."
Vijayan alleged that it was a pre-planned attack and CPI-M would not succumb to the "sangh parivar's threat."
"No one should think that CPI-M can be cowed down or silenced with violence," he said.
The chief minister also alleged that the sangh parivar under the cover of BJP-led government at the Centre has "unleashed a fascist model violence across the country."
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The chief minister's office tweeted "strongly condemn the attack on Com: @SitaramYechury.It amounts to an attack on Indian Democracy."
Opposition leader Ramesh Chennithala said the attackers were "cowards" who could not challenge ideology with ideology and that is why they confront political rivals through violence.
Chennithala alleged that Hindu sena activists attacked the CPI-M leader and it was a challenge to democracy.
CPI state secretary Kanam Rajendran condemned the incident and said it exposed the 'intolerant attitude' of the Sangh Parivar.
BJP in Kerala also condemned the 'attack' and said it was inappropriate to physically attack a political leader in a democratic country.
Two Hindu sena workers have been arrested in connection with it, he said, adding, "they have told police that CPI-M leaders' unsavoury remarks against army had provoked them."
"Still it is with an ulterior motive that efforts are on to link BJP-RSS with the incident...CPI-M should retreat from that attempt," he said.
Rajasekharan said CPI-M leaders had not come forward even to condemn the attack on the BJP office here few months ago."BJP now condemns the attack on CPI-M leader as the party believed in democracy," he added.
KPCC president M M Hassan said confronting political rivals through violence is a challenge to democracy and wanted strong action to be taken against the culprits.
Condemning the incident, veteran CPI-M leader V S Achuthanandan said government should take steps to ban outfits like the Hindu Sena.
Two persons were held today for allegedly barging into the CPI(M) head office in Delhi and trying to manhandle Yechury.
The incident took place when Yechury was about to enterthe central committee hall on the first floor of the CPI(M) office to address the press on conclusion of the party's two-day politburo meeting.
However, the Left alleged the duo owed allegiance to an RSS outfit and entered the office "masquerading as journalists" in an attempt to disturb the press conference, which was later held as scheduled.