* "Unknown" claims No. 1 with $21. 8 million
* "I Am Number Four" takes No. 2 with $19.5 million
* Final figures await end of U.S. Presidents Day holiday (Adds other titles, details; byline)
LOS ANGELES, Feb 20 (Reuters) - Thriller "Unknown" topped movie box office charts with $21.8 million in ticket sales during a sluggish weekend over the long Presidents Day holiday in the United States.
"Unknown," starring Liam Neeson as a man who must reclaim his identity after finding it stolen when he awakens from a coma, bested another new release, sci-fi adventure "I Am Number Four," which landed in the No. 2 spot with $19.5 million, according to film studio estimates on Sunday.
Animated "Gnomeo & Juliet" rounded out the top three on Sunday, earning $19.4 million for a scant drop of only 23 percent from its debut last weekend. Typically, a major studio release like "Gnomeo" might be expected drop 40-50 percent in its second weekend, but the family film has been helped by generally good critics' reviews and audience ratings.
Box office estimates will likely change by Tuesday when U.S. moviegoers return to work after a long holiday weekend that includes Presidents Day.
By Sunday, the weekend's No. 4 movie was comedy "Just Go With It" starring Adam Sandler and Jennifer Aniston. Its ticket sales dropped 40 percent to $18.2 million from last weekend and now total $60.8 million in the United States and Canada.
Last week's No. 2 movie, "Justin Bieber: Never Say Never," a look at the life of teenage singing sensation Bieber, saw a decline of 54 percent and fell to No. 6 with 13.6 million. Its total domestic revenues number $51.7 million after two weeks.
The weekend's other new major release, Martin Lawrence comedy "Big Mommas: Like Father Like Son" claimed the No. 5 spot with a $17 million debut.
Ticket sales for all films in theaters from Friday through Sunday were estimated at $144 million, down around 30 percent from 2010's Presidents Day weekend tally of $204.6 million, according to industry tracker Hollywood.com Box Office.
Other notable releases included Oscar hopeful "The King's Speech," which landed at No 7. with $6.6 million and pushed its total box office up to $103.3 million.
Rounding out the top 10 were holdovers from recent weeks: "The Roommate," "The Eagle" and "No Strings Attached" in the No. 8, 9 and 10 spots, respectively.
"Unknown" was released by Warner Bros, a unit of Time Warner Inc (TWX.N). "I Am Number Four" was produced by privately held DreamWorks and distributed by the studio unit of The Walt Disney Co (DIS.N), while "Gnomeo and Juliet" was co-produced and distributed by studio units of Walt Disney.
"Big Mommas: Like Father Like Son" was released by 20th Century Fox, a division of News Corp (NWSA.O). "Just Go With It" was released by Columbia Pictures and "The Roommate" was released by Screen Gems. Both are units of Sony Corp (6758.T).
"The Eagle" was released by Focus Features, part of Universal Pictures that is 51 percent-owned by Comcast Corp (CMCSA.O). "Justin Bieber: Never Say Never" and "No Strings Attached" were released by Paramount Pictures, a unit of Viacom Inc VIAb.N, and "The King's Speech" was released by the Weinstein Co, which is privately held. (Editing by Eric Walsh) (For a chart on top movies at the North American box office, click on [ID:nN20208420])