The deceased included two children and two women as their houses on the slopes of the hills in Idukki district were swept away by flash floods, police said.
A district where most people are small and marginal farmers, Idukki also suffered heavy crop and property loss.
Several persons were reported missing as a huge mound of rock and rubble came crashing down at Chiyyappara in Idukki while relief work was on in the area where landslip occurred in the early hours.
Several vehicles were trapped in the debris and rescue work hampered as roads the district got blocked by rubble spawned by landslide and flashfloods.
The Cochin International Airport has been temporarily closed and two flights diverted as the runway got waterlogged. Periyar river and canals are overflowing after the downpour lashed the area since Sunday.
"Cochin International Airport has been closed till 2.30pm as water entered the runway as the Periyar river is overflowing after shutters of the Edamalayalar dam were opened," airport director ACK Nair said.
The sandbanks of the Periyar river in Aluva, where thousands are to converge to perform the Balitharpanam ritual on Tuesday on the day of Karkitakavavu, is flooded and the Shiva temple there almost submerged.
Ernakulam district collector Sheikh Pareed said possibilities of making alternative arrangements for the conduct of the ritual were being explored and the help of Navy sought to tackle the situation.
People in affected areas have been shifted to relief camps.
Chief minister Oommen Chandy has called an emergency cabinet meeting in the state capital to review the serious situation and gear up the relief operations.
The IMD has cautioned fishermen not to venture into the sea in the next 48 hours as strong winds of 45-55 kmph could lash coastal areas along with heavy rain.
With agency inputs.