The announcement was made after a meeting here of the leaders of the parties including JD(U), Left, Samajwadi Party, AIADMK.
However, the ticklish issue of their Prime Ministerial candidate was put off till after the elections.
Addressing a press conference after the hour-long meeting, CPI(M) General Secretary Prakash Karat said the 11 parties will work together to defeat UPA whose rule has witnessed massive corruption and price rise.
Moreover, BJP represents communal politics which is dangerous for the country, he added.
The 11 parties wil present an alternative to UPA and BJP, said the CPI(M) leader.
He appealed to other "secular" parties to join them in their endeavour.
Mulayam Singh said the third alternative grouping could expand in future. "Today, we are 11 parties, it can be 15 parties later," he said.
