Gilbert Baker, an artist best known for creating the rainbow flag as a symbol for the gay community, has died aged 65.
The San Francisco-based activist died in his sleep on Thursday night at his New York home, friends said.
A candlelit vigil was due to be held in his honour on Friday night in San Francisco.
Longtime friend Cleve Jones broke the news on social media.
My dearest friend in the world is gone. Gilbert Baker gave the world the Rainbow Flag; he gave me forty years of love and friendship. pic.twitter.com/titd3XZ0zD— Cleve Jones (@CleveJones1) March 31, 2017
He tweeted: "My dearest friend in the world is gone.
"Clive Baker gave the world the rainbow flag, he gave me forty years of love and friendship."
San Francisco friends, meet me under Gilbert's flag tonight 7pm Castro/Market. pic.twitter.com/yPCk4jrXBX— Cleve Jones (@CleveJones1) March 31, 2017
Mr Jones also tweeted a photo of Baker with former President Barack Obama, inviting mourners to meet him under a rainbow flag in the Castro district of San Francisco on Friday evening to remember his friend.
Mr Baker, who was born in Kansas in 1951, was stationed in San Francisco in the early 1970s while serving in the US Army, at the start of the gay rights movement.
He began making banners for gay rights and anti-war protests, often at the request of Harvey Milk, who would become the first openly gay man elected to public office in California when he won the 1977 race for a seat on the San Francisco Board of Supervisors.
Mr Milk rode under the first rainbow flags made by Baker at the San Francisco Gay Freedom Day Parade in June 1978, just months before the politician was murdered by a former city supervisor, the biography says.
Speaking about the flag in an interview with the New York Museum of Modern Art, which exhibited the flag in 2015, he said: "I decided that we should have a flag, that a flag fit us as a symbol, that we are a people, a tribe if you will.
"And flags are about proclaiming power, so it's very appropriate."