Vijayan said the attack amounted to an assault on Indian democracy.
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-M has decided to hold protests across the state, condemning the incident, party sources said.
Two persons allegedly barged into the CPI(M) head office in Delhi and shouted slogans against the Left, ahead of a press conference by Yechury.
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