Published: Apr 26, 2010 11:39 AM EDT

PHOENIX (AP) - Activists are calling on President Barack Obama to fight a tough new Arizona law they say will lead to police harassment of legal immigrants and U.S. citizens who look Hispanic.

U.S. Rep. Raul Grijalva of Arizona told about 3,500 protesters at the state Capitol Sunday that the Obama administration can help defeat the law by refusing to cooperate with its implementation.

The law requires Arizona police officers to question people about their immigration status if there's reasonable suspicion they're in the country illegally. Officers would arrest people found to be undocumented and turn them over to federal immigration officers.

Opponents say the federal government can block the law by refusing to accept them.