Many users were unable to access their sites and services on Monday.
The attack started at about 10:45am in New York, according to the company's status page. The company said the "evolved" attack, a distributed denial-of-service (DDoS) attack, affected almost every region around the globe -- including Asia and the Americas.
By mid-afternoon, the company was able to stabilize its systems after several configuration changes to mitigate the attack, describing it as a "defensive posture."
But the attack persisted throughout the day, with further disruption hitting networks and end users into late-evening in Europe.
Lewis said that the attack was "one of the largest and most sophisticated we have ever observed," with "many tens of millions of packets hammering our network every second, complex migration of traffic across the network, and a series of precise strategies for targeting our systems."
NS1 did not give specific figures of how large the attack was.
However, we've noted in our previous coverage that industry sources are aware of attacks matching 600 Gbps which have been previously detected and privately reported. Attacks that big are rare, and are understood to be difficult to carry out, but they aren't impossible.
No group or malicious actor has publicly taken credit for the attack.