The US government has officially shut down for the second time this year because Congress failed to meet a deadline to vote on a new budget.
Senators struggled with last-minute objections from Republican Rand Paul, but have now passed the bill, which has gone to the House for its vote.
Federal funding for government services expired at midnight (05:00 GMT).
The 600-page plan proposes an increase in spending, by about $300bn (£215bn), on defence and domestic services.
If the plan is passed in the House of Representatives and signed by the president in the next few hours, the shutdown could be rescinded before the US working day begins on Friday.
But it is not clear how the House will vote, and how public services would be affected on Friday if the shutdown were to continue.
Many government agencies close during a shutdown as their future funding is theoretically not secure. Many employees are asked not to come to work and will not be paid - although some will get back pay.
Employees deemed essential - including military personnel and air traffic controllers - are required to work regardless of shutdowns.
Three weeks ago, some people lost three days of work in a shutdown but this time, it is not yet clear which agencies will close.
The federal Office of Personnel Management said employees should "refer to their home agency for guidance on reporting for duty".
CNN is reporting that if the shutdown is not averted, government agencies will still be able to call their employees in for a half day's work to make the shutdown go smoothly.
Some Twitter users shared stories of how the uncertainty would affect people.
Why are budget hawks opposed to the bill?
While the spending bill's funding for the Pentagon has delighted the national security wing of the party, fiscal conservatives are up in arms about ramifications for the nation's debt.
In a doom-laden speech, Senator Paul angrily charged his fellow Republicans with fiscal profligacy, accusing his colleagues of "spending us into oblivion".
"I ran for office because I was very critical of President Obama's trillion-dollar deficits," he said.
"Now we have Republicans, hand in hand with Democrats, offering us trillion-dollar deficits.
"I can't in all good honesty, in all good faith, just look the other way just because my party is now complicit in the deficits."
This would be "the very definition of hypocrisy", he added.
What's in this bill?
The 650-page spending plan was only unveiled on Wednesday night, so the finer details are unclear.
White House legislative affairs director Marc Short said the package would increase spending by "just shy" of $300bn.
The Washington Post puts the figure at half a trillion dollars.
The bill contains $165bn of additional defence spending and $131bn in domestic spending, including funding for healthcare, infrastructure and tackling the US opioid crisis, reports Reuters news agency.
The proposal would raise the US debt ceiling until March 2019.
Why are some Democrats unhappy?
Despite the support of their Senate leader Chuck Schumer, who says the budget accord will "break the long cycle of spending crises", some Democrats have complained that the bill does not address immigration.
The party's leader in the House of Representatives, Nancy Pelosi, said on Thursday morning she was opposed to the plan, but would not order rank-and-file Democrats to vote against it.
The California congresswoman has called for the bill to include a provision shielding so-called Dreamers, young immigrants who entered the US illegally as children, from deportation.
Her remarks came a day after she told the stories of immigrants for eight hourson the floor of the lower chamber in a record-breaking speech.
Obama-era guarantees for those immigrants were cancelled by US President Donald Trump and are set to become invalid next month.
Illinois representative Luis Gutierrez, one of the leading congressional advocates for immigrants, is urging colleagues to vote against the plan.
"Don't collude with this administration," he said.
Haven't we been here before?
Yes. Apart from this year, the US government has closed for business several times before.
In 2013 a shutdown lasted for 16 days, after Republicans demanded the spending bill have provisions to impede or delay President Barack Obama's Affordable Care Act.
National monuments and parks were closed and hundreds of thousands of government workers were put on unpaid leave. Only one person was left to patrol the 5,525 mile (8,891km) border with Canada.
But in most cases of shutdowns, the House and Senate are controlled by opposing parties. This time, as was the case three weeks ago, both chambers of Congress plus the White House are controlled by the Republicans.