Hi,
As Richard says, the problem is probably due to MTNL's irritating habit of injecting Ads. I have never seen them do a full page one - i.e. usually I saw small popups - but I have had the URLs blocked for a couple of years now, so I suppose they have "progressed".
Regarding Rony's question "Is there any setting in my router that can reduce this?". Yes. You need to block some URLs/IPs on your router. My list is as follows: 203.94.227.141:3000 203.94.227.141:80 203.94.227.140:80 203.94.243.40:80 adphonso.com:80 phozeca.com:80 Where the setting will be depends on your router - on my D-Link it's "Advanced Setup" => "URL Filter"