Police and downtown commuters braced Wednesday for clogged streets and blocked intersections as tens of thousands of immigration supporters planned to march in annual May Day demonstrations.

Los Angeles police will post traffic officers at several downtown intersections on Hill and Spring streets as demonstrators march north on Broadway toward Olvera and 3rd streets.

Rally participants were expected to begin gathering early in the morning, with the largest marches expected to build up in the afternoon. Occupy Los Angeles posted on its Facebook wall that it will join in the demonstrations.

Many of the demonstrations are assembling at Olympic Boulevard and Broadway.

At least 30,000 people are expected to participate.

The Los Angeles Department of Transportation posted a list of DASH detours for Wednesday, starting at 11:30 a.m. 

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Joseph.serna@latimes.com

@josephserna