Charter97.org is available to me, and very conveniently, fully in English (why would Belarussians set up an opposition site in English?) The site is hosted in the USA. The other sites are hosted in Russia and they seem indeed to be down.
The Belarussian site "ale.by"is up and showing pictures of a crowd with some red-white banners on a square. No riots, no police violence. Compare with education reforms in the UK.
Maybe the Russian language opposition sites are just getting more hits than they can handle, due to the elections. It's pretty common for sites to go down when lots of people legitimately try to hit them at a same time, due to some specific event.
Anyone who believes that a somebody with a genuinely held political view is going to change their mind because a couple of websites are down, is plain stupid. There are about a million ways around any such blocks anyway, and normally there is no restriction of internet usage in Belarus.
If the sites were hosted in Belarus, then the authorities could take them down. But since they are not, there isn't anything anyone can do, apart from DDosing them a bit, or maybe blocking them at the largest ISPs. This is extremely easy to spot, since you'd get a completely different site, and wouldn't get to the site you are trying to reach at all. Here, you DO get the site but it is busy - either legitimately or by DDOSing.
If that is really being done by Belarussian intelligence, then it's certainly wrong, fairly pathetic and most of all a miracle the regime has lasted so long, since it would be imbecill to waste time on something like that. I hardly think it's anything they are involved in.
EDIT 1:
ROFL- Guess who this lady voted for?
EDIT 2: While I was on the Belarussian opposition website, I got a virus?!?!!! Huh??! First time ever, that I got one just from being on a website, with Firefox. Not sure which site it was. Luckily I had a clone of my C drive, so no big harm done.