By Ted Hesson

WASHINGTON Immigration advocates represented by the American Civil Liberties Union (ACLU) filed a legal challenge on Thursday minutes before a U.S. regulation restricting access to asylum at U.S.-Mexico border was set to take effect.

In a motion filed before a federal judge in California, the groups want to re-open an existing lawsuit over similar restrictions implemented by Republican former President Donald Trump and update the case to target the new regulation issued by President Joe Biden, a Democrat.

The groups said the Biden regulation "dramatically curtails the availability of asylum in the United States" and mirrored similar Trump-era policies blocked in court.

The strict new asylum regulation is a key part of Biden's strategy to address a possible rise in illegal immigration as COVID-19 restrictions, known as Title 42, ended at midnight on Thursday.

Tens of thousands of migrants have crossed the U.S.-Mexico border illegally in recent days, with some saying they were worried Biden's new regulation would be more strict.

(Reporting by Ted Hesson in Washington; Editing by Clarence Fernandez)