The US National Security Agency lost its authority to collect phone records in bulk, with intelligence officials warning it could jeopardise Americans' safety.
A midnight local time deadline passed after a dramatic Sunday session in the Senate, when Republican Senator Rand Paul stood in the way of extending the fiercely contested programme.
The programme and several other post-September 11, 2001 counter-terror measures look likely to be revived within days.
With no other options, Senate Majority Leader Mitch McConnell reluctantly embraced a House of Representatives-passed bill that would extend the provisions, while also remaking the bulk phone collections programme.
Civil liberties groups applauded as Mr Paul, who is running for president, forced the expiration of the once-secret programme made public by NSA contractor Edward Snowden. Critics call it an unconstitutional intrusion into Americans' privacy.
The Senate voted 77-17 to move ahead on the House-passed bill, the USA Freedom Act.
The White House backs the House bill. White House press secretary Josh Earnest said: "The Senate took an important - if late - step forward tonight. We call on the Senate to ensure this irresponsible lapse in authorities is as short-lived as possible."
But the Senate adjourned without final action on the bill after Mr Paul asserted his prerogative under Senate rules to delay a final vote for a couple of days. The midnight deadline came and went.
He said: "This is what we fought the revolution over, are we going to so blithely give up our freedom?"
His actions greatly complicated matters for Mr McConnell, who has endorsed him for president, and infuriated fellow Republicans. They exited the Senate chamber en masse when Mr Paul stood up to speak.
Senator John McCain complained that Mr Paul places "a higher priority on his fundraising and his ambitions than on the security of the nation".
In addition to the bulk phone collections provision, two lesser-known Patriot Act provisions also lapsed when the deadline passed.
One, so far unused, helps track "lone wolf" terrorism suspects unconnected to a foreign power. The second allows the government to eavesdrop on suspects who continually discard their mobile phones.
The House bill extends those two provisions unchanged, while revising the bulk collection programme so the NSA would stop collecting the phone records after a six-month transition period, but would be authorised under court order to search records held by phone companies.
The FBI's use of the Patriot Act to collect hotel, travel, credit card, banking and other business records in national security investigations would also be extended under the House bill.
Law enforcement officials say the collection of those business records is more valuable than the better-known bulk phone collections programme.
Ongoing investigations would be permitted to continue even though authority for the programmes has lapsed.
Rebooting the phone collections programme would take about a day.
CIA director John Brennan was among those warning that letting the authorities lapse, even for a time, will make America less safe.