Rajappan, a CPI-M activist hailing from Perumala in Nedumangad near here, fell down following uneasiness at the venue of the protest near Assembly complex in the morning.
He was rushed to the government general hospital, where doctors declared him brought dead, police said.
He is survived by his wife and two children.
The protest was organised by the CPI(M)-led LDF in an attempt to prevent Mani from presenting the state budget over the bar bribery scam. The protesters also demanded his resignation.
The agitation against Mani also turned violent outside the House with police using batons and teargas shells to chase away angry LDF and Yuva Morcha activists.