San Francisco is apparently in lockdown as a gunman armed with a semiautomatic weapon is in standoff with police.

A SWAT team and the bomb squad were called to the United Nations plaza after reports of an armed gunman, according to US media reports.

The man was reportedly seen holding a gun outside City Hall, with authorities urging locals to "avoid the area".

More than 100 police had surrounded the area, according to witnesses.

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