In Bengal's Dhupguri, CPM and Congress have joined forces to take on the Trinamool Congress.
The Opposition has come out on top, albeit by a very narrow margin, in what was being seen as a major test for it after the formation of the INDIA alliance. Parties that are members of the bloc have won in four of the seven state Assembly seats where bypolls were held on Tuesday.
While BJP candidates have been elected from the Dhanpur and Boxanagar assembly seats in Tripura and Bageswhar in Uttarakhand, the Opposition bloc has won in Uttar Pradesh's Ghosi, Jharkhand's Dumri, Puthuppally in Kerala and Dhupguri in West Bengal.
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Dara Singh Chauhan, the sitting MLA from Ghosi assembly constituency in Uttar Pradesh and a popular OBC leader, had rejoined the BJP after resigning from the Samajwadi Party in July. His resignation necessitated a by-election in the seat, which has been won by the Samajwadi Party's Sudhakar Singh with a margin of 42,759 votes.
Delivering a big blow to the ruling CPI(M), Chandy Oommen of the Congress-UDF alliance won the Puthuppally by-election in Kerala by a margin of over 36,000 votes. Mr Oommen, son of late Congress stalwart Oommen Chandy, gained a clear edge in the first round and retained that position till the counting of votes was completed.
Jharkhand Mukti Morcha (JMM) candidate Bebi Devi has won the by-election to the Dumri assembly seat in Jharkhand's Giridih district and defeated All Jharkhand Students Union (AJSU) party nominee Yashoda Devi by over 17,000 votes.
Over in West Bengal, two INDIA bloc parties - the Congress and the CPI(M) - joined forces to take on their ally, the Trinamool Congress (TMC), in the BJP-ruled Dhugpuri constituency. However, the TMC's Nirmal Chandra Roy prevailed and asserted the Mamata Banerjee-led party's control over the seat.
In Tripura, BJP's Tafajjal Hossain won the Boxanagar seat while the tribal-dominated Dhanpur was won by Bindu Debnath.
In Uttarakhand's Bageswhar, after taking an early lead over the BJP's Parwati Dass, the Congress party's Basant Kumar fell behind. Ms Dass defeated Mr Kumar by over 2,400 votes. The bypoll was held after the death of four-time MLA and cabinet minister Chandan Ram Das in April.
Of the seven seats that went to polls, Dhanpur, Dhugpuri and Bageshwar were held by the BJP. Ghosi was held by the Samajwadi party while the seat in Jharkhand was held by the JMM. Tripura's Boxanagar seat and Kerala's Puthuppally were with the CPM and the Congress, respectively.