The mural in Dallas, Texas, painted to commemorate 50th anniversary of Stonewall riots. (NBCDFW)

A mural commemorating the Stonewall Riots – painted to honour transgender icons Marsha P Johnson and Sylvia Rivera, who were key figures in the protests – has been defaced by vandals.

Police in Texas are looking for people who vandalised the mural, which features LGBT+ activists Marsha P Johnson and Sylvia Rivera.

Dark blue aerosol paint was used to paint moustaches on the upper lips of both Johnson and Rivera.

The mural was painted by New York artist Brian Kenney to commemorate the 50th anniversary of the Stonewall uprising, and is the largest mural in the world honouring the trans community, according to non-profit Artitude, which creates and supports platforms for LGBT+ people to represent their identities, histories, and communities.

The protests at the Stonewall Inn in New York City happened in June 1969 as a response to a police raid against the LGBTQ+ patrons of the Inn.

The protests are often credited as the beginning of the modern gay rights movement. The new mural in Cedar Springs shows two women, Marsha P Johnson and Sylvia Rivera, who were part of the Stonewall Riots and are thought of as two pioneers of the gay rights movement.

“We will start cleaning the mural by Wednesday when the graffiti cleaner arrives,” said Jerome Larez from Artitude.

Larez added that they are going to see if they can remove the graffiti without damaging the mural.

The graffiti was first discovered on December 8, according to the Dallas Voice.

Artitude said in a statement: “The popular transgender mural commemorating the 50th anniversary of Stonewall was defaced over the weekend.

“Dark blue aerosol paint was used to deface the mural by painting on the upper lips of both Marsha P. Johnson and Silvia Rivera.

“The mural intends to bring the LGBTQ+ community together and to promote visibility for the transgender community by recognising Marsha P Johnson and Silvia Rivera, both trans women of colour and the unsung heroines of Stonewall.”