Ruptures in the opposition unity emerged barely a week after 17 parties joined hands to put up a joint candidate against the ruling NDA nominee for the July 17 presidential election.
The Congress, the Trinamool Congress, the Left, the BSP, the DMK, the RJD and some other parties boycotted the event, dubbing it as a "tamasha" (drama).
Anwar said his party was attending the GST launch as it had supported the legislation on ushering in the new indirect tax regime in the country.
"We agree with the Congress to an extent, but we supported the legislation when it was introduced (in Parliament and state assemblies). The party has hence decided to attend the midnight meeting," he said.
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JD(S) supremo and former Prime Minister H D Deve Gowda also attended the programme, sharing the dais with President Pranab Mukherjee, Vice President Hamid Ansari, Prime Minister Narendra Modi, Lok Sabha Speaker Sumitra Mahajan and Finance Minister Arun Jaitley.
Yesterday, JD(S) general secretary Danish Ali had said that Gowda had been invited in the capacity of former prime minister to share the dais with the President and the party had decided that he would attend the event.
The JD(S) criticised the Congress for not consulting it before taking the decision to boycott the function.
The SP, which had initially announced the boycott, did a somersault by attending the event, saying it found no problem when the party had supported the GST bill in Parliament.
The Congress, however, downplayed the cracks within the opposition ranks, terming it as a "one-off" development.
A senior Congress leader claimed that the opposition unity will remain intact during the 2019 Lok Sabha election.
The function was attended by the union ministers, MPs as well as chief ministers and finance ministers of several states.