Some campers have left, the crowd has grown with tentless sympathizers, and there has been milling in the traffic lanes of 1st Street at Spring, inhibiting passing cars but no actual trouble. Police presence: light — but the LAPD has reportedly declared a citywide tactical alert. All via media reports, web streams and Twitter (I'm not there.)
* 12:20 update: Reporters have spotted police in riot gear on Temple Street and at 1st and Broadway, blocking protesters from walking west. Channel 4 back on the air live as crowds move into the street and LAPD vehicles begin to appear. (Channel 4 went back to some celebrity show after a few minutes, then came back on live from City Hall.)
12:38 a.m.: 1st Street now completely blocked by Occupy participants between City Hall and LAPD headquarters, the block from Spring to Main. Some members are reportedly trying to get people to go back into the park. The LAPD has asked media trucks on Main Street to move south of 1st Street, which I'm guessing would put them outside a perimeter around City Hall.
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Web screen grab: KTLA.com