The demonstrations led by opposition members and youth activists are a significant expansion of the unrest sparked by the Tunisian uprising, which also inspired Egypt's largest protests in a generation. They pose a new threat to the stability of the Arab world's most impoverished nation, which has become the focus of increased Western concern about a resurgent al Qaeda branch, a northern rebellion and a secessionist movement in the south.
Crowds in four parts of Sanaa have shut down streets and are chanting calls for an end to the government of President Ali Abdullah Saleh, who has been in power for nearly 32 years.
"We will not accept anything less than the president leaving," said independent parliamentarian Ahmed Hashid.
Opposition leaders called for more demonstrations on Friday.
"We'll only be happy when we hear the words 'I understand you' from the president," Hashid said, invoking a statement issued by Tunisian President Zine El Abidine Ben Ali before he fled the country.
Saleh has tried to defuse simmering tensions by raising salaries for the army and by denying opponents' claims he plans to install his son as his successor.
After the Tunisian turmoil, he ordered income taxes slashed in half and instructed his government to control prices. He deployed anti-riot police and soldiers to several key areas in the capital, Sanaa, and its surroundings to prevent riots.
That hasn't stopped critics of his rule from taking to the streets in days of protests calling for him to step down, a red line that few dissenters had previously dared to cross.
Nearly half of Yemen's population lives below the poverty line of $2 a day and doesn't have access to proper sanitation. Less than a tenth of the roads are paved. Tens of thousands have been displaced from their homes by conflict, flooding the cities.
The government is riddled with corruption, has little control outside the capital, and its main source of income - oil - could run dry in a decade.
Saleh's current term in office expires in 2013 but proposed amendments to the constitution could let him remain in power for two additional terms of ten years.
U.S. Secretary of State Hillary Rodham Clinton urged Yemen to step up security cooperation with the United States during an unannounced visit this month to shore up ties.
Following the Obama administration's pattern in Afghanistan and Pakistan, Clinton also emphasized that the United States wanted a broader relationship with Yemen beyond the fight against violent extremists. Clinton was the first U.S. secretary of state to visit Yemen in two decades.
Radicals have used the country as a base for launching attacks on the U.S. The radical cleric Anwar al-Awlaki, thought to be hiding in Yemen, is suspected of having inspired some of those attacks.
Clinton said the U.S. supports efforts to address the underlying causes of extremism: poverty, corruption, social inequality and political divisions that have boiled into an insurgency. She said Yemen must stop the practice of child marriage and enact reforms.
In the past five years, U.S. military assistance to Yemen has totaled about $250 million. In 2010, military and civilian aid was almost evenly split and combined for about $300 million.
Military aid to Yemen would reach $250 million in 2011 alone, U.S. officials said, and Clinton said there will be additional development aid.
Yemen has been the site of numerous anti-U.S. attacks dating back to the 2000 bombing of the USS Cole in Aden harbor, which killed 17 American sailors
Just last month, several CIA operatives were the targets of a failed bombing at a restaurant in a Sanaa suburb, and Al Qaeda in the Arabian Peninsula was thought to be behind the attempted bombing of an American airliner landing in Detroit on Christmas Day 2009.
Al-Awlaki is thought also to have inspired the deadly 2009 shooting rampage at Fort Hood, Texas. The al Qaeda group's fighters attacked the U.S. Embassy in Sanaa twice in 2008.
With the help of U.S. money and training by elite U.S. commandos, Yemen is setting up provincial anti-terrorism units to confront al Qaeda in its heartland.