UPDATE 1-Rothko painting defaced at Tate Modern in London

(Adds arrest)

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

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

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

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

Rothko, a key figure in 20th century abstract painting whoseworks sell at auction for tens of millions of dollars, wascommissioned to paint the Seagram Murals in the 1950s for theFour Seasons restaurant in New York.

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

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

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

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

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

((Mike.Collett-White@thomsonreuters.com)(tel: +44 7990 560229))