LOS ANGELES -- At a Home Depot parking lot, a man patrols on a bicycle for federal immigration agents, toting a megaphone on his hip so he can blast a warning to day laborers waiting to land a landscaping or construction job.

The workers from Mexico, El Salvador and elsewhere carry whistles to also sound the alarm, while activists swap details over two-way radios about whether cars whizzing by could be unmarked vehicles carrying officers preparing for a raid.

Their work is cut out for them. Agents have raided the lot outside the 108,000 square-foot Home Depot store in the Van Nuys neighborhood of Los Angeles at least five times this summer, rounding up some immigrants and sending others running in search of safety.

Home Depot stores in Southern California have long been an informal j

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