The massive human chain, which according to the Karnataka government will be the "world's longest", is being formed across the state from Bidar to Chamarajanagar, covering all 31 districts.
The state government has taken the lead in organising the large-scale event to mark the day in association with civil society.
Chief Minister Siddaramaiah joined the human chain at the main event held in front of Vidhana Soudha, the seat of state legislature and secretariat here, along with senior Ministers and officials.
The CM led by reading the preamble of the Constitution, following which people from various walks of life, including transgenders and differently-abled people held hands to form the human chain.
Similar events were held in all districts of the state in which Ministers, public representatives and senior officials took part.
A world record verifying team from London will arrive for the innovative and massive event, officials said.
The human chain will be approximately 2,500 km long, and is said to be the "longest in the history of the world".
A total of 25 lakh people are expected to participate, with an average of more than 1,000 people for each kilometre, the government has said.
During the event, 10 lakh saplings are expected to be planted in the state by the participants, it added.
According to officials, the participants of the human chain will be given certificates.
Last year, the government organised a mega event of reading the
Since 2007, September 15th has been declared as the International Day of Democracy, which is celebrated globally. Proclaimed by the United Nations (UN), the day serves as a platform to promote and uphold the principles of democracy worldwide.