Members of the UDF cabinet and former Chief Minister and CPI(M) stalwart V S Achuthanandan were among those who took oath.
The Congress-led UDF secured power in the assembly polls held in April, narrowly defeating CPI(M)-led LDF by winning 72 seats in the 140-member house.
Pro-Tem Speaker N Sakthan (Congress) presided over the proceedings. The Speaker of the assembly will be elected tomorrow.
The UDF has proposed senior Congress legislator G Karthikeyan for the post and the LDF has decided to put up a fight by fielding former minister A K Balan (CPI-M).