Confusion has persisted, however, over the number of blasts. Initial media reports said a series of explosions had struck the prayer meeting minutes after it began.
Videos from the convention centre showed people screaming and running, with toppled chairs as a fire swept parts of the convention centre.
A state minister, VN Vasavan, also claimed at least two consecutive blasts took place.
“It is an unusual accident. All agencies are here for preliminary examination,” said Mr Vasavan.
India’s federal counter-terrorism unit, the National Security Guard (NSG), has rushed one of its bomb disposal units from national capital Delhi to Kerala to collect materials from the blast site and conduct an investigation.