PHOENIX — A national motor-vehicle computer network malfunctioned Monday, preventing Arizona’s online voter-registration system from being used on the day leading up to the deadline to register to vote in the state’s presidential primary.

Midnight Monday was the deadline for registering for the Feb. 5 primary.

It wasn’t known how many Arizonans were unable to use the online registration system because of the unexplained problem with the national network. But more than 5,000 either registered or changed their registrations over the weekend, a state election official said.

Under Arizona law, only Democrats and Republicans can vote in their respective party’s presidential primary, though independents can vote in the state’s regular primary election in September.

Independents are a growing political force in Arizona, constituting just over 28 percent of the electorate.

Elections officials advised voters wanting to change their registrations to visit county recorder offices, and the Arizona Secretary of State’s Office planned to keep a drop-off station open until midnight. Mailed registration forms, available online on the Secretary of State’s Web site, http://www.azsos.gov, also would be valid if postmarked by Monday.

Mesa resident Trevor Johnson, who was a registered independent, went to the state’s drop-off site late Monday after trying unsuccessfully all day to re-register online so he could vote in the Republican primary.

“Technology’s wonderful but when it’s not working, it’s frustrating for sure,” Johnson said.

The computer malfunction occurred in a national network operated by the Arlington, Va.-based American Association of Motor Vehicle Administrators. Arizona and other states use the network to check Social Security numbers and other data for a variety of government functions.

“It was discovered early this morning. The reports began coming in first thing this morning,” said Cydney DeModica, spokeswoman for the Arizona Motor Vehicle Division. “This is a nationwide issue.”

Jason King, a vice president and spokesman for the national group, confirmed that the association’s network “experienced system problems that impacted the flow of information across the network.”

King said the result was a “slowdown,” not a complete stoppage, but multiple attempts to use Arizona’s online registration system on Monday drew automated responses that it was temporarily unavailable. Arizona Deputy Secretary of State Kevin Tyne said he couldn’t immediately determine whether the network was allowing any Arizona voter registrations to be processed online Monday.

The cause of the problem wasn’t immediately known and it wasn’t immediately known when it would be fixed, King said in an e-mail statement.

“On behalf of its members, the state motor vehicle agencies, AAMVA apologies for any inconvenience this has caused customers in transacting their DMV business.”

Political party officials said their offices’ phones were jammed with calls from voters wanting to be able to cast ballots in the Feb. 5 election.

“We’ve had all five lines blinking,” said Sean McCaffrey, Arizona Republican Party executive director.

Democratic Party spokeswoman Emily Bittner said recent news coverage of the inability of independents to vote in the presidential primary prompted a recent flood of inquiries.

“We have had literally thousands of calls, e-mails and drop-ins,” Bittner said.