A federal judge in Washington ruled on Wednesday that the Trump administration cannot categorically deny entry to people crossing the southern border to claim asylum, striking down a change made on President Trump’s first day in office.

The ruling rejected the idea, repeatedly put forth by the president, that such extraordinary powers were justified to curtail what Mr. Trump has called an invasion of the United States by immigrants crossing the southern border.

In a hefty 128-page opinion, Judge Randolph D. Moss of the Federal District Court for the District of Columbia wrote that the Constitution and federal immigration law did not afford Mr. Trump the expansive authorities he claimed.

“The court recognizes that the executive branch faces enormous challenges in preventing and deterring unlawful entry into the United States and in adjudicating the overwhelming backlog of asylum claims of those who have entered the country,” he wrote.

But neither the Constitution nor the current law governing asylum seekers, Judge Moss wrote, could “be read to grant the president or his delegees authority to adopt an alternative immigration system.”

Despite the sweeping terms of the order, Judge Moss postponed them from taking effect for two weeks, to give the Trump administration time to appeal.