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* Twitter messages rally support for opposition
* Websites are focal point for young Iranians
* U.S. State Dept. asked for Twitter outage delay
(adds State Dept. comment, paragraphs 5-7)
By Georgina Prodhan
LONDON, June 16 (Reuters) - Supporters of defeated Iranian presidential candidate Mirhossein Mousavi posted defiant messages on Twitter on Tuesday, calling for a second banned pro-Mousavi rally to go ahead and offering security updates.
Social networking websites such as Twitter and Facebook have become a focal point for young, urban Iranians opposed to President Mahmoud Ahmadinejad, who defeated Mousavi in Friday's presidential election and whose government controls the state media.
"Alert: Mousavi march still on. 5PM," read one short message, or "tweet," on Twitter.
"Good luck at the march. Don't take cars, they will be waiting for you when you return to them," read another, as tweets on the subject of the Iranian election poured onto the site every few seconds.
A U.S. official said meanwhile that the U.S. State Department had contacted Twitter at the weekend to urge it to delay a planned upgrade that could have cut daytime service to Iranians.
"We highlighted to them that this was an important form of communication," the official said of the conversation the department had with Twitter. The official, speaking in Washington, declined further details. The request was made despite the Obama administration's stated concern not to meddle in the post-election dispute.
Twitter had said earlier it had delayed the upgrade, without mentioning any contact with the U.S. government.
The Iranian government blocked SMS text messages during polling after opposition candidates used them to galvanise key young voters during the fiercely contested election campaign, and Tehran residents were still unable to send SMSs on Monday.
The BBC's Persian service was also blocked.
Social networking site Facebook, which has about 150,000 members in Iran, said on Monday it had had reports that some users in Iran were having difficulties accessing Facebook.
"This is disappointing, especially at a time when citizens are turning to the Internet as a source of information about the recent election," it said in a statement.
Twitter Inc said in a blog post on its site it was delaying a planned upgrade because of its role as an "important communication tool in Iran." The hourlong maintenance was put back to 5 p.m. EDT/2100 GMT, which corresponds to 1:30 a.m. on Wednesday in Iran.
It said the upgrade had orginally been planned for Monday night in the United States, which would have cut daytime service in Iran on Tuesday.
Unrest has rocked Tehran and other cities since the Interior Ministry released results on Saturday that showed hardliner Ahmadinejad had defeated Mousavi by a landslide.
Mousavi has appealed to Iran's top legislative body to annul the result because of irregularities, a charge the Interior Ministry and Ahmadinejad have dismissed. The Guardian Council said it was ready to carry out a partial recount.
According to Internet censorship monitor OpenNet Initiative, Iranians began taking to the Internet in droves during the 1997-2005 presidency of Mohammad Khatami, when dozens of independent publications were shut down and journalists jailed.
In a 2007 report on Iran, the OpenNet Initiative estimated there were about 400,000 blogs in Farsi, as publishing on the Internet exploded, despite what it called "one of the most extensive technical filtering systems in the world."
More than 23 million Iranians in a country of 70 million -- more than 60 percent of whom are under the age of 20 -- have access to the Internet.
Facebook was shut off in Iran on May 23, joining political and human-rights websites which had already been blocked. The ban was lifted on May 26, following strong criticism from moderate candidates.
Mousavi has almost 48,000 supporters on his main Facebook page. Ahmadinejad also has a page with 2,615 fans, while the "I bet I can find 1,000,000 people who dislike Mahmoud Ahmadinejad!" group has more than 57,000 members. (Additional reporting by Claudia Parsons in New York and Sue Pleming in Washington; Editing by Giles Elgood and Frances Kerry)