Here are the top 10 developments of the story:
Raids were held at over 30 places across the state, including properties of Mr Vijayabaskar - the state health minister - and his associates in other cities.
Officials say Rs 2 lakh was seized from Mr Vijayabaskar's home, including "money kept for buying voters". Around five crores was the total haul from the minister and his associates, said an official.
Mr Vijayabaskar has denied the allegations. "Officials have not recovered even Rs 10,000 from me. IT officials harassing me... they didn't even let my children go to school," said the minister.
Over 100 taxmen along with security personnel were part of the operation. The home of Geetha Lakshmi, the Vice Chancellor of the Dr MGR Medical University, was also searched.
The raid team said they were acting on "inputs" that black or untaxed money is being pumped into the by-election.
The ruling party's faction led by VK Sasikala alleges that Mr Vijayabaskar and Sarath Kumar have been targeted because of their support to its candidate TTV Dinakaran. The Sasikala camp also blamed the ruling BJP at the centre, accusing it of ordering the Income Tax raids for political gains, in collusion with the OPS camp.
The raids have been linked to a video shared widely on social media earlier this week, which showed a man handing out four thousand rupees to three people while urging them to vote for TTV Dinakaran's hat symbol,
O Panneerselvam or OPS's faction the AIADMK Purathchi Thalaivi Amma has fielded veteran leader E Madhusudanan against Mr Dinakaran. Ms Jayalalithaa's niece Deepa Jaykumar is also contesting the by-election. All three stake claim to the legacy of Ms Jayalalithaa or Amma.
Last night, at least three persons were injured in clashes between supporters of Sasikala and Panneerselvam. Mr Panneerselvam's group caused a commotion earlier in the day by using a replica of the coffin of Ms Jayalalithaa for its campaign.
The BJP denied any political motive behind the raids, saying it was only an "administrative" action by the sleuths.
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