WASHINGTON (AP) — Civil liberties groups on Wednesday asked for a temporary restraining order blocking the Trump administration's effort to effectively end asylum at the U.S.-Mexico border.

The American Civil Liberties Union and others filed the request seeking a Thursday hearing in San Francisco. The groups sued Tuesday and want the judge to block the policy while the case is heard. A second lawsuit is pending in Washington, D.C.

Hundreds of immigrants have showed up at border crossings in hopes of getting into the U.S. But the new rules that went into effect earlier this week bar most migrants from seeking protection as refugees if they have passed through another country first. It targets tens of thousands of Central Americans who cross into the U.S. through Mexico, but also affects people from Africa, Asia, and South America who come to the southern border.

It was the most forceful attempt to date by President Donald Trump to slash the number of people seeking asylum in America. It comes at a time when Trump's recent tweets telling four members of Congress to "go back" to other countries have set off an uproar.

U.S. officials have said the plan would discourage migrants from leaving their countries, which is necessary to reduce the numbers of people border agents are detaining.

There are some exceptions: If someone has been trafficked, if the country the migrant passed through did not sign one of the major international treaties that govern how refugees are managed (though most Western countries have signed them) or if an asylum-seeker sought protection in a country but was denied — then a migrant could still apply for U.S. asylum.

But immigration advocates say the Trump administration's action is illegal because it circumvents the process Congress has established for asylum. They compared it to the Trump administration's earlier attempts to ban anyone crossing the southern border illegally from seeking asylum, which a federal court stopped.

U.S. asylum law allows refugees to request asylum when they arrive regardless of how they did so, but there is an exception for those who have come through a country considered to be "safe."

At the northern border, the U.S. and Canada have a "safe third country" agreement that limits people who arrive in one country from seeking asylum in the other. The U.S. has sought to reach the same agreement with Mexico and Guatemala, but nothing has been decided.

The Trump administration's treatment of Central American migrants has come under heavy criticism as border officials manage record numbers of people crossing the border. Advocates and Democratic lawmakers have reported fetid conditions in overcrowded border facilities, with children trying to care for themselves and images of packed cells where some detainees don't have room to sleep.

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Associated Press Writer Nomaan Merchant contributed to this report from Houston.