This incident is caused by ISP provider CenturyLink who suffered from a global BGP routing outage, one of the largest on internet so far that Cloudflare saw a 3.5% drop in global traffic during the outage. The NOC operation at CenturyLink took more than four hours to complete the problematic BGP Flowspec announcement thus leading to the global internet outage.
Please see below for the root cause confirmation we received from CenturyLink:"The major ip issue we faced, was due to a problematic Flowspec announcemen which prevented Border Gateway Protocol (BGP) from establishing correctly.
The IP NOC deployed a configuration change to block the offending Flowspec announcement, thus restoring services to a stable state."