A civil rights group filed a federal lawsuit Tuesday to force Virginia to extend its voter registration period after the state’s online system crashed Monday, the last day to register, preventing an unknown number of voters from getting on the rolls.

One registrar estimated that “tens of thousands” of Virginians had been unable to register by the cutoff at 11:59 p.m. Monday, although the state elections commissioner, Edgardo Cortés, said the number was unknown.

The meltdown prompted a Washington-based civil rights group to file the lawsuit on behalf of Kathy and Michael Kern, a Charlottesville couple who tried multiple times Sunday and Monday to register without success. Two nonprofit groups involved in voter-registration drives — New Virginia Majority Education Fund and Virginia Civic Engagement Table — also are plaintiffs.

“The right of eligible Virginians to vote should not be victim to a technological failure that is no fault of their own,” said Kristen Clarke, president of the Lawyers’ Committee for Civil Rights Under Law, which is representing the plaintiffs along with attorneys from the Washington firm Arnold & Porter.

The suit, filed Tuesday night in U.S. District Court in Alexandria, names Cortés and other state election officials as defendants.

But Cortés said the deadline is specified in state code, which provides no option for an extension. “We don’t have any legal authority to extend the registration deadline,” he said.

Clarke said her group would file a complaint in federal court to compel the state to extend its registration window.

Since the last presidential election, Virginia has rolled out a system that allows citizens to register to vote and update their registrations online. Four years ago, the state had an entirely paper-based system.

The shift to online was hailed as a more modern and efficient approach, but registrars have complained in recent months about recurring glitches and severe slowdowns. Problems resurfaced Sunday, after Facebook and Google alerted Virginians to the state’s looming registration deadline.

“The Department of Elections online voter registration system is experiencing an unprecedented activity level that has caused it to slow down and sometimes be completely unresponsive,” the department said on its Facebook page. Visitors to the page were advised to “wait a few minutes and try again.”

It was no use. The problems worsened Monday, even after Cortés directed registrars around the state to stop using the system to free up capacity for citizens trying to log in.

“It got inundated to the point where it was useless for everyone,” said Greg Riddlemoser, registrar in Stafford County.

His office received hundreds of phone calls on Monday from voters unable to register online. He estimated that the number of citizens who had been unable to register statewide was in the “tens of thousands.”

It is not clear how many voters tried and failed to register through the system because there is no record of those who could not complete the task, Cortés said.

But judging by the drop-off in registrations, the number appeared to be in the thousands in some large jurisdictions alone, such as Fairfax County.

In Fairfax in recent weeks, as many as 6,000 people a day have registered online as new voters or updated their addresses, Registrar Cameron Sasnett said. On Monday, the number was less than half that: 2,729. Not a single person in Fairfax succeeded in accessing the system after 7 p.m., he said.

Sasnett called the situation “chaos wrapped inside an implosion.”

Speaker William J. Howell (R-Stafford) criticized state-level elections officials for the troubles.

“We have every confidence in Virginia’s local registrars, but we are concerned that Governor [Terry] McAuliffe’s Department of Elections has barely recognized, let alone responded to, the registrars’ concerns about the technical systems needed to administer the elections,” he said. “The administration needs to take immediate corrective action to ensure that local registrars have the necessary technical and support resources for the November election.”

Sasnett put part of the blame on procrastinating citizens.

“The system itself failed, but this wasn’t the only opportunity or mechanism for somebody to register,” he said. “We had 364 more days to register as well.”

The state, which has 5.5 million voters on its rolls, has seen last-minute registration spikes in other presidential years. Just under 430,000 Virginians registered in October 2012. While results for this month are not known yet, the spike seems even sharper this time around amid a bitterly contested race between Republican Donald Trump and Democrat Hillary Clinton. More than 490,000 Virginians registered in September, compared with 260,000 in September 2012.

“I don’t think the system was quite ready for prime time,” ­Riddlemoser said. “I’m not sure why we didn’t predict we would get crushed.”

The state launched its online voter registration system in 2013. Until this year, the largest number of registrations submitted in a single day was about 2,200, Cortés said. That single-day online registration record has been broken three times this year in response to social-media campaigns urging Virginians to register: 8,000 signed up one day in February, ahead of the deadline to register for the primary; 17,000 on Sept. 23, ahead of National Voter Registration Day; 21,000 managed to do so on Monday, despite the day’s technical issues.

“This level of activity for our system is just unprecedented,” Cortés said. “As a state agency, we don’t have the ability to just scale up.”