Cyberattacks on a key internet firm repeatedly disrupted the availability of popular websites across the United States, according to analysts and company officials.
The White House described the disruption as malicious.
Members of a hacker group claimed responsibility, although their assertion couldn't be verified.
New Hampshire-based Dyn Inc said its server infrastructure was hit by distributed denial-of-service attacks, which work by overwhelming targeted machines with junk data traffic.
The attack had knock-on effects for users trying to access popular websites from across the US and even in Europe, affecting sites such as Twitter, Netflix and PayPal.
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The level of disruption was difficult to gauge, but Dyn provides internet traffic management and optimisation services to some of the biggest names on the web, including Twitter, Netflix and Visa.
Critically, Dyn provides domain name services, which translate the human-readable addresses such as "twitter.com" into an online route for browsers and applications.
Steve Grobman, chief technology officer at Intel Security, compared an outage at a domain name services company to tearing up a map or turning off GPS before driving to the department store.
"It doesn't matter that the store is fully open or operational if you have no idea how to get there," he said in a telephone interview.
Jason Read, founder of the internet performance monitoring firm CloudHarmony, owned by Gartner Inc, said his company tracked a half-hour-long disruption early on Friday (US time) in which roughly one in two end users would have found it impossible to access various websites from the US east coast. A second attack later in the day caused disruption to the US east and west coasts as well as impacting some users in Europe.
"It's been pretty busy for those guys," Read said. "We've been monitoring Dyn for years and this is by far the worst outage event that we've observed."
A full list of affected companies wasn't immediately available, but Twitter, Netflix, PayPal and the coder hangout Github said they briefly experienced problems earlier on Friday.
Members of a shadowy hacker collective that calls itself New World Hackers claimed responsibility for the attack via Twitter. They said they organised networks of connected "zombie" computers that threw a staggering 1.2 terabits per second of data at the Dyn-managed servers.
"We didn't do this to attract federal agents, only test power," two collective members who identified themselves as "Prophet" and "Zain" told an AP reporter via Twitter direct message exchange. They said more than 10 member participated in the attack. It was not immediately possible to verify the claim.
The collective, NewWorldHacking on Twitter, has in the past claimed responsibility for similar attacks against sites including ESPNFantasySports.com in September and the BBC on December 31. The attack on the BBC marshalled half the computing power of Friday's onslaught.
The collective has also claimed responsibility for cyberattacks against the Islamic State group. The two said about 30 people have access to the NewWorldHacking Twitter account. They said 20 are in Russia and 10 in China. "Prophet" said he is in India. "Zain" said he is in China. The two claimed to be taking "good actions".
Another collective member the AP previously communicated with via direct message called himself "Ownz" and identified himself as a 19-year-old in London. He told the AP that the group - or at least he - sought through hacking only to expose security vulnerabilities.
