CAIRO, Egypt — Governments in the Middle East are stepping up a campaign of censorship and surveillance in order to block an estimated 33.5 million Internet users from viewing websites whose topics range from adult entertainment to human rights.

As a result, millions of Middle Easterners are having trouble accessing news and entertainment sites like Facebook, MySpace, YouTube and Flickr.

The prohibitions have led to an explosion in "circumventors," proxy servers that allow Internet users to bypass workplace or government filters. In cyber cafes throughout the Middle East, patrons still can browse blocked sites and swap web addresses for the latest "proxies."

Five of the Top 13 Internet censors worldwide are in the Middle East, according to Reporters Without Borders, a Paris-based journalism advocacy group that lobbies against web censorship.

Only four Arab countries have little or no filtering: Lebanon, Morocco, Jordan and Egypt. On the other side of the web censorship gap are Saudi Arabia and Syria, which have consistently been described by human rights groups as the most hostile toward the Internet.

Authorities in Syria continue to ban websites, including Amazon.com last month. The government reportedly uses a filtering system called Thundercache to block content from sites such as Blogspot, Hotmail, Skype and YouTube, as well as any Arabic-language news sites.

In Iraq and the Palestinian territories, the Internet is policed mainly by the owners of Internet cafes and by Internet users themselves. Islamist militants have reportedly attacked Internet cafes in both places, accusing patrons of looking at adult material or chatting with members of the opposite sex.

In Iraq, the only official Internet censor is the U.S. military. Operational security measures stop American troops from using some sites, and commanders have shut down cyber cafes in areas where insurgents use the Internet to share intelligence and plan attacks.

Nonelectronic censorship also is spreading throughout Arab states in North Africa. Tunisian authorities block several sites, human rights workers said, but the authorities also have started holding the owners of Internet cafes liable if political activists use their establishments to post critical news about the government.

In Egypt, the Arab world's most populous nation and home to an estimated 6 million Internet users, the government offers cheap dial-up browsing to anyone with a telephone line and authorities do little or no filtering.

Video-sharing platforms, social-networking sites, most opposition sites and adult material all are easily accessible, but police have rounded up at least three bloggers and harassed many more in recent years, according to Reporters Without Borders.

Iran's hard-line Shiite Muslim leadership also is a zealous censor of the Internet. The government boasts of filtering 10 million "immoral" websites, in addition to all the major social-networking outfits and dozens of pages about religion or politics.

Additionally, the ultraconservative Saudi government blocks thousands of adult websites.