In the Panaji contest, Chief Minister and BJP stalwart Manohar Parrikar is in the fray against Congress vice-president Rahul Gandhi's confidante Girish Chodankar.
In Valpoi, the BJP has fielded Congress rebel and Health Minister Vishwajeet Rane, whose main opponent is Roy Naik, the son of former state Home minister Ravi Naik, of the Congress.
Panaji has been a bastion for BJP since 1994.
The polling, to be held at 76 booths including 30 in Panaji and 46 in Valpoi, will take place between 8 am to 5 pm.
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North Goa Collector Nila Mohanan said that adequate security arrangements have been made to ensure that the polling passes off peacefully.
"Around 1,000 personnel from agencies like the Goa police, India Reserve Battalion and Central Paramilitary Forces are deployed at various locations," she said.
The counting of votes will take place on August 28.
The Panaji bypoll was necessitated after the BJP's sitting MLA Siddharth Kuncolienkar vacated the seat for Parrikar, as the latter has to get elected to the Assembly within six months of becoming the Chief Minister.
While Parrikar has promised the overall development of Panaji through an Action Plan, the Congress alleged that the BJP leader, who had represented the constituency from 1994 to 2012, has proved "inefficient".
In Valpoi seat, Rane, son of veteran Congress leader and former chief minister Pratapsinh Rane, has assured of creating employment for the local youth.
In the Assembly elections, the Congress won 17 seats and the BJP 13 in the 40-member House.
However, the saffron party formed the government under Parrikar on the backing of three MLAs each of the Goa Forward Party (GFP), Maharashtrawadi Gomantak Party (MGP), and three Independents.
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