malicious alert notification

As of a few days ago I get on my iMac (MacOs Ventura 13.6.1) each and every some minutes an alert-notification: hacking attempt, loss of data, data theft attempt, critical hardware error, etc.... (each time a different message) inviting me to click on the message to solve the issue.

When clicking a safari-window opens with a Norton-AV design asking to click on an install button to solve the problem (I didn't do of course), next time it was a black browser window inviting me to establish a VPN connection (I didn't do either).

I run AVG-antivirus (latest update, full paid version) and he does not detect this.

I did a hard-restart of my Mac but the message persist appearing.

Even when my system has no open apps/webpages/.... these messages appear

I tried to follow the procesactivities but it goes too fast to be able to follow

These messages appear just for a few seconds, and the disappear again.

What can I do to get rid of them?