[Windows Central Question]

When I switch on my phone the screen shows a message stating that a 'CRITICAL UPDATE' is required. There is no sign saying that it's from Microsoft or Nokia and no reason why this update is required.

[RumoredNow]

It is a software and/or firmware update. The OS itself would block an "update" from an unknown source.

[Zahin Wadud]

I have also got a "Critical Update" a few minutes earlier. And it's downloading now. Under - Settings > Phone Update - I can see the progress. So, is that an OS update? Lumia Denim? Or just some error fixing? I have never got this kind of message before.
And Mr. Guest, who also have got this thing, what happened in your case? Did you have an OS update?
Thanks in advance.

[RumoredNow]

I have also got a "Critical Update" a few minutes earlier. And it's downloading now. Under - Settings > Phone Update - I can see the progress. So, is that an OS update? Lumia Denim? Or just some error fixing? I have never got this kind of message before.
And Mr. Guest, who also have got this thing, what happened in your case? Did you have an OS update?
Thanks in advance.

Welcome to the site, Zahin.

The OP is indeed a guest, an unregistered guest. These persons do not see a reply button and have no way of responding to your question.

I believe you know by this point if your update brought you Denim. I think it did.

Again, welcome!!!