The letter quoted one estimate as saying that one Wal-Mart supermarket in India would displace over 1,300 small retail stores and render about 3,900 people jobless.
"In a situation when employment growth has slowed down, the entry of foreign supermarkets would further aggravate the employment situation," it stated.
"We urge the government not to open up the retail trade to FDI any further. Political parties across the spectrum are opposed to this move. Many state governments have also stated their opposition," it said.
The letter was signed by Samajwadi Party leader Mulayam Singh Yadav, CPI-M's Prakash Karat, CPI's S Sudhakar Reddy, Danish Ali of Janata Dal-Secular, Debabrata Biswas of Forward Bloc and Abani Roy of Revolutionary Socialist Party.
The letter pointed out that the Indian retail sector was the second largest employer in India after agriculture.
"It employs over four crore people. Most of these are small unorganized or self-employed retailers," it stated.