A historic Black church in Memphis, Tennessee, that was an organizing point for Martin Luther King's last civil rights campaign caught fire early on Monday morning and was badly damaged.

The inside of the Clayborn Temple was gutted by flames, leaving only exterior walls standing, Memphis Fire Chief Gina Sweat told a press conference.

Police were investigating the cause of the blaze reported at 1:39 a.m., she said.

A monument with the words "I Am A Man" remained standing outside the church, marking its role as the headquarters from which Black sanitation workers marched in a 1968 strike to protest inhumane working conditions. Many carried placards bearing that slogan.

King traveled to Memphis that year to lead a march and was assassinated the day before on April 4, 1968.

"Clayborn is more than a historic building. It is sacred ground. It is the beating heart of the civil rights movement, a symbol of struggle, hope, and triumph that belongs not just to Memphis but to the world," Memphis Mayor Paul Young wrote on social media.

At the time of the blaze the church was undergoing a $14 million restoration that included creation of a museum and a performance space, according to the Clayborn Temple website.

Listed in the National Register of Historic Places, the Romanesque Revival church was built in 1892 and originally served an all-white congregation. It was sold to the African Methodist Episcopal Church in 1949 and renamed Clayborn Temple after their bishop.