UPDATE 1-Rothko painting defaced at Tate Modern in London

(Adds arrest)

LONDON, Oct 8 (Reuters) - A man was arrested on Monday on suspicion of defacing a painting by the Russian-American artist Mark Rothko in London's Tate Modern museum.

The 26-year-old was detained at about 9 p.m. in connection with a case of criminal damage, London's Metropolitan Police said.

The words "Vladimir Umanets, a potential piece of yellowism" were daubed in black onto the bottom-right corner of the painting, "Black on Maroon", on Sunday.

The popular gallery, located in a converted brick power station on the River Thames, issued a statement confirming that "a visitor defaced one of Rothko's Seagram murals by applying a small area of black paint with a brush".

Rothko, a key figure in 20th century abstract painting whose works sell at auction for tens of millions of dollars, was commissioned to paint the Seagram Murals in the 1950s for the Four Seasons restaurant in New York.

Several of them ended up in the Tate collection after being given as gifts, and Tate describes the series of soft-edged, coloured rectangles as "iconic".

Tim Wright wrote on the Twitter micro-blogging site that he had witnessed the incident and posted a photograph of the damaged canvas online.

"Very bizarre, he sat there for a while then just went for it and made a quick exit," Wright wrote. "This guy calmly walked up, took out a marker pen and tagged it. Surreal."

In May, Rothko's "Orange, Red, Yellow" sold for $86.9 million, a new auction record for the artist, at Christie's in New York.

(Reporting by Mike Collett-White; Editing by Kevin Liffey)

((Mike.Collett-White@thomsonreuters.com)(tel: +44 7990 560 229))