Users of the popular online payment processing service PayPal have reported major problems logging in or making transactions — a critical time as Cyber Monday sales are in full swing. 

Twitter was full of complaints from merchants and customers.   

There was no mention of the outage on PayPal's Twitter account and a request for comment from PayPal was not immediately answered.

But on PayPal's merchant notification page, there was an alert of a "major impact" to PayPal APIs (application program interfaces) and its website.

The website downdetector.com said a heat map of PayPal's problems showed widespread outages affecting users in North America and Europe. 

The website downrightnow.com indicated that the outage began around 8:30 a.m. ET and lasted at least an hour. 

The U.S. department store chain Target was also reporting problems with its website on Monday morning. It blamed heavy Cyber Monday traffic.

Visitors to target.com were met by an apology graphic that read: "So sorry, but high traffic's causing delays. If you wouldn't mind holding, we'll refresh automatically & get things going ASAP." 

Target's website appeared to be working normally as of noon ET.

The chain had previously advertised that virtually every item on its website would be 15 per cent off on Monday.

Website outages or slow checkouts during the five-day U.S. Thanksgiving online shopping season were also reported at Neiman Marcus, Wal-Mart Stores and Foot Locker, according to Reuters.

The U.S. National Retail Federation estimates that 121 million Americans were planning to shop online on Cyber Monday.

With files from Reuters