"We have spent our careers working for the security of our country. Collectively, our group holds no affiliation with any single political party, our only common commitment being to the Constitution of India," the letter signed by 114 veterans of all three forces said.
"We stand with the 'Not in My Name' campaign that mobilised thousands of citizens across the country to protest against the current climate of fear, intimidation, hate and suspicion," it said.
"What is happening in our country today strikes at all that the Armed Forces, and indeed our Constitution, stand for... We are witness to unprecedented attacks on society at large by the relentless vigilantism of self-appointed protectors of Hinduism," the letter said.
"We condemn the targeting of Muslims and Dalits... We condemn the clampdowns on free speech by attacks on media outlets, civil society groups, universities, journalists and scholars, through a campaign of branding them anti-national," it added.
"We can no longer look away... Our diversity is our greatest strength. Dissent is not treason; in fact, it is the essence of democracy," the letter said.
The letter comes a month after thousands hit the streets in cities across India to protest a surge in mob violence. The campaign called 'Not In My Name' was sparked by the killing of a 16-year-old Muslim boy was killed by a mob on a train near Delhi who accused him of being a 'beef-eater'.
Prime Minister Narendra Modi has also condemned the killings, saying, "Killing people in the name of Gau Bhakti (reverence of cows) is not acceptable."