Protests over the cartoons continue in Pakistan
Instructions were issued to internet service providers across Pakistan on 27 February to block about a dozen websites of various origins.
The ban comes amid protests in several Muslim countries against the cartoons, first published in Denmark last year.
Islamic tradition prohibits depictions of the Prophet.
Hosted site
Bloggers in Pakistan first became aware of the ban on 28 February when they were unable to access a popular blog hosting site, Blogspot.
One of the blocked sites is hosted on Blogspot, which led to the blocking of all web journals hosted on the site.
The Pakistan bloggers found their blogs blocked, even though their blogs are not connected with the cartoons.
They say they have still been able to edit and update their blogs, but not able to read them.
Blogspot, which hosts web logs, has been blocked
Many are using anonymizers - a service which allows people to surf sites without being identified - to access their sites, while the US blogging community has sent out suggestions for ways in which Blogspot can be accessed.
The BBC's Aamer Ahmed Khan in Karachi says the blocking is unlikely to turn into a major freedom of expression issue in Pakistan as there seems to be a consensus against allowing such freedom to extend to irreverent treatment of religious figures.
Pakistani bloggers agree the blocking of Blogspot cannot be intentional, but are hoping the authorities realise the problem soon and rectify it, he said.
There are many ways of blocking specific sites without letting it affect the entire hosting facility, bloggers say.
Pakistan, meanwhile, is seeking an internationally applicable law against blasphemy.
Pakistani bloggers have been reporting this week that the Pakistani government has started blocking local access to the blogging tool Blogger.com. Blogger.com, along with its companion domain blogspot.com, is one of the most widely used blogging tools in Pakistan. Because of the government's decision to block the site, many Pakistani bloggers are now unable to update their blogs.

Bloggers are debating whether the move by the Pakistani government is a sign of a long-term online censorship campaign, or a temporary move to prevent potentially embarrassing blog coverage of U.S. President George W. Bush's visit to the country this weekend.

"Truth is — it may have very little to do with Bush's visit," writes Indian blogger Neha Viswanathan on the Global Voices email list, run by the Berkman Center for Internet & Society at Harvard Law School. "The anti-Bush protest has hardly found a voice online. I really do think this might be the first of other blocks. This might be on a very experimental basis to see how far they go. The Pakistani govt has hardly had any dialogue with bloggers at all. "

"The other theory is of course the one on Danish cartoons- which is likely," she continues. "But there is so much content on that outside of the blogosphere (the limited one at Blogspot) that it doesn't' seem to fit together. I wouldn't be surprised if the govt of Pakistan comes up with an IT censorship policy."

Other bloggers see a more direct connection with the Bush visit. "I have communicated with several bloggers and friends back in Pakistan," writes UAE-based Pakistani blogger Omer Alvie. "So far there's no resolution to this problem. Bush's visit in Pakistan is resulting in protests, a strike in Lahore and curfew areas in Islamabad. The feeling among bloggers is this is all rather suspect. Neha, might be right this might be on experimental basis. I feel this a a ''sic'' precursor to what the government can do in the future."

To date, the Pakistani government has offered no comment on the decision to block online access to Blogger.com.