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LONDON — A British decision to bar a visit by two prominent American anti-Islam activists has prompted righteous indignation from the targets of the ban as well as a more measured debate about the limits of free speech.

Pamela Geller and Robert Spencer, right-wing bloggers and co-founders of Stop Islamization of America, on Wednesday received letters from Theresa May, Britain’s home secretary responsible for interior affairs, telling them that their presence would “not be conducive to the public good.”

The pair had been due to address a rally on Saturday of the far-right English Defense League (E.D.L.) in the London suburb of Woolwich, where Lee Rigby, a British soldier, was killed in a vicious daylight attack on May 22.

Their association with the E.D.L. stems from the British group’s past support for their campaign against the so-called “Ground Zero Mosque,” a planned mosque and community center near the site of the World Trade Center in Lower Manhattan.

The decision to keep them out was based on the likelihood they would use their visit to repeat what were regarded as their unacceptable and provocative statements about Muslims that might foment violence.

Ms. Geller, a self-styled human rights activist, wrote on her blog, Atlas Shrugged, that her only crime was her “principled dedication to freedom.”

Mr. Spencer, who runs the Jihad Watch Web site, said the British decision amounted to a ban on those who “oppose jihad violence and Islamic supremacism.”

Ms. May took her decision in the context of a reported increase in attacks on mosques and other Islamic centers since Drummer Rigby’s murder, for which two Muslims have been charged and which the E.D.L. is accused of exploiting in order to fuel anti-Islamic sentiment.

News of the ban coincided with reports that a mosque in the English town of Redditch was broken into and daubed with Swastikas, racist slogans and the letters “EDL.”

As Ms. Geller proclaimed that Britain had killed the Magna Carta, a government spokesman was quoted by the BBC as saying, “We condemn all those whose behaviors and views run counter to our shared values and will not stand for extremism in any form.”

The ban followed a campaign by Britain’s anti-fascist Hope Not Hate movement against allowing the pair into the country.

Matthew Collins, a researcher with the organization, welcomed the decision and said, “They only wanted to come here and help the E.D.L. stir up more trouble. Britain doesn’t need more hate even just for a few days.”

Keith Vaz, an opposition Labour Party legislator, also welcomed Ms. May’s decision, saying, “The U.K. should never become a stage for inflammatory speakers who promote hate.”

However, James Bloodworth, a left-wing British commentator, warned of the risk of turning Ms. Geller and Mr. Spencer into “free speech martyrs.”

Writing before the ban was announced, he said it would be counterproductive.

“These people already self-pityingly style themselves as a silent majority whose opinions are repressed by the state,” he wrote at the Left Foot Forward Web site. “Do we really want to provide them with such easy ammunition to throw back at us?”

“Banning them for what they might say is also dangerous territory,” he wrote. “Free speech within the law should be paramount.”

Ben Cohen, a British contributor to the American magazine Commentary, said the ban reflected double standards on the part of a country that had put up with the presence of Islamic extremists.

“One does not have to an admirer of Geller and Spencer — to my mind, their views are terrifyingly shrill and bigoted — in order to consider this decision outrageous,” he wrote.

“Banning Geller and Spencer will not mollify those British Muslims already on the path to self-radicalization,” Mr. Cohen wrote. “Nor is it likely to end the disturbing spate of attacks on mosques in the wake of Lee Rigby’s killing.”

The main result of the ban was to make British democracy look weak and spiteful at precisely the time it needed to look strong and confident, he concluded.

What do you think? Is Britain right to want to keep out potential troublemakers? Does a desire to preserve public order sometimes trump the right to free speech?