Islamabad, Pakistan - Pakistan's officially declared "Day of Love for the Prophet Muhammad" devolved into deadly violence in major cities Friday as tens of thousands of Pakistanis angrily demonstrated against an Islam-mocking YouTube video, although calm generally prevailed in other predominantly Muslim countries.

At least 20 people died and more than 150 were injured in the protests in Pakistan, authorities said - the highest one-day death toll since protests began over the video on Sept. 11. The demonstrations have spread to about 20 nations.

The government's announced effort to tamp down anger by providing a national holiday for peaceful protest clearly backfired, offering instead what seemed like an official sanction to violence.

Critics called the holiday a pandering attempt to please hard-line Islamist parties, whose influence has been on the rise here in recent years.

"This was a terrible idea," said Mehreen Zahra-Malik, a columnist with the News, a national English-language daily. "It was time to calm people down and not give a stamp of approval to protesters, many of whom would just use it as an excuse for violence. . . . There was clearly going to be violence."

Another commentator, Marvi Sirmed, said on Twitter: "It is sad, so very sad that we could never make a government realize that they don't have to kneel before mullah," a reference to Muslim clerics.

Despite repeated U.S. disavowals of the privately made video and denunciations of its content, many Pakistanis remained unconvinced, seeing it as an intentional calumny against the prophet Muhammad.

Most of the fatalities and destruction came in the southern port city of Karachi, where Saghir Ahmed, health minister for Sindh province, said 14 people died, including two policemen shot by rioters. At least 80 people were wounded, Roshan Ali Shaikh, the city's police commissioner, said.

In the northwestern city of Peshawar, rescue workers and other officials said six people were killed, including a policeman and a member of a television crew, in rampages that also left about 60 people wounded. Television journalists on the scene said police opened fire with live rounds as mobs torched two movie houses.

Demonstrators also battled security forces for the second day in the usually calm capital, Islamabad, in the north. They blocked major highways there and in neighboring Rawalpindi and set a tollbooth and vehicles on fire.

Fourteen police officers were injured in the chaos, Interior Minister Rehman Malik said. The Pakistani army was mobilized and successfully protected the U.S. Embassy, presidential residence and Parliament building.

In the eastern city of Lahore, officials said 12 riot police officers and four protesters were injured during pitched battles involving thousands of demonstrators.

The rioters in all four cities targeted U.S. diplomatic facilities but failed to reach them, thwarted by Pakistani police and paramilitary forces who had set up barbed-wire barricades and steel shipping containers to deter demonstrators.

On Friday evening, as the death toll continued to climb, Prime Minister Raja Pervez Ashraf renewed his calls for peace.

"Destroying property and resorting to violence negate the spirit of Islam and teachings of the prophet," Ashraf said in a statement.

Other countries that had previously seen violence connected to the YouTube video were largely calm Friday. In Kabul, where a suicide car bomber earlier this week killed 12 people in an attack linked to the video, Friday prayers passed without incident.

As he had done the previous Friday, Afghan President Hamid Karzai called on clerics to discourage protests.

In Cairo, the scene last week of an assault on the U.S. Embassy over the video, just a few dozen people stood in front of the shuttered French Embassy to protest cartoons showing a nude Muhammad that were published by the French satirical weekly magazine Charlie Hebdo.

Egypt's highest Islamic legal official said Thursday that Muslims should endure insults without resorting to violence. The country's main Islamist parties did not call for embassy protests.

But in Pakistan, where anti-American sentiment runs high, the protests gained momentum through the day.

In Karachi, officials and journalists at the scene said marchers torched movie theaters, banks, American food franchises and police vehicles and pelted the Sheraton hotel with stones.

Hard-line Islamist parties, including two banned factions, joined the throng, estimated to number 15,000.

Abdul Ghani, a Karachi shopkeeper, said the security forces opened fire on the protesters and turned the demonstration violent. "We were completely peaceful and just wanted to register our protest in front of the U.S. Consulate," he said. "That unwanted and uncalled-for action by the police got the mobs infuriated."

The chief mufti of Pakistan, Rafi Usmani, unavailingly urged the crowds to remain calm. "If you burn your property, vehicles, banks and police stations, whose purpose are you serving?" he said. "We all are with you to protest, but it must be peaceful."

Earlier, addressing a gathering of clerics, the prime minister said Muslim nations should press the United Nations and other international bodies to outlaw blasphemy. "Our demand is simple: Blasphemy of the kind witnessed in this case is nothing short of hate speech, equal to the worst kind of anti-Semitism or other kinds of bigotry," Ashraf said, according to Pakistan's state news agency.

In anticipation of Friday's protests, authorities closed gas stations and jammed cellphone service in at least 15 cities across the country. Earlier, Pakistan had blocked access to YouTube, where the incendiary video was posted.

The Obama administration has purchased ads on a half-dozen Pakistani television stations to disavow the video. The $70,000 ad buy demonstrates the depth of U.S. concern about the volatility of Pakistan, where several militant jihadist groups, including the Taliban, are based in tribal areas and operate largely free of Islamabad's authority.

Pakistan's Foreign Ministry summoned the U.S. Embassy's charge d'affaires, Richard Hoagland, on Friday to demand that the United States take immediate measures to remove the video, purportedly a trailer for a film titled "Innocence of Muslims," from YouTube.

"This was an attack on 1.5 billion Muslims and a premeditated and a malicious act to spread hatred and violence among people of different faiths," ministry officials told Hoagland, according to a statement.

Leiby reported from Kabul. Haq Nawaz Khan in Peshawar, Nisar Mehdi and Aamir Latif in Karachi and William Booth in Cairo contributed to this report.