|A Yemeni protester waves a dagger as he shouts slogans in support of President Saleh in Sanaa [AFP]
Thousands of protesters have taken to the streets across Yemen for the fourth straight day, demanding political reforms and the downfall of Ali Abdullah Saleh, the country's long-serving president.
The 3,000-strong throng of demonstrators in the capital, Sanaa, comprising students, human rights activists and lawyers clad in black robes, clashed with police and pro-government supporters on Monday.
Rival groups, armed with clubs and rocks, were seen facing off after supporters of Saleh reportedly confronted the protesters.
At least three people were injured, including one stabbed with a traditional Yemeni dagger, in fighting outside Sanaa's university where protesters chanted: "A revolution of free opinion ... A revolution of freedom ... We should be allowed to decide."
Further chants of "After Mubarak, Ali" and "No corruption after today" reverberated around the city.
"It's very tense on the streets here," Al Jazeera's Hashem Ahelbarra, reporting form Sanaa, said.
"The anti-government protests are gathering momentum, despite the huge concessions given by President Ali Abdullah Saleh, who said that he will step down in 2013, and that his son, General Ahmed Abdullah Saleh - who was being groomed to take over - won't inherit power.
"Thousands of young Yemenis are saying: 'We won't trust Ali Abdullah Saleh, because we trusted him in the past, particularly in 2006, when he said he was about to resign - only for him to continue and to ask for the constitution to be amended to allow him to be appointed for life'."
Public squares blocked
Protesters have been using social media networks, such as Facebook and Twitter, in an attempt to mobilise people throughout Yemen, an impoverished country at the south of the Arabian peninsula.
But state forces have blocked access to public squares, several coincidentally named "Tahrir Square" - both in Sanaa, and around the country.
The move was an apparent attempt to prevent the world's media from linking the protests with those that took place in the Egyptian capital, Cairo, our correspondent says.
Several checkpoints have appeared on streets leading to Sanaa's presidential palace, and many have been blocked with razor wire.
"What we are seeing is thousands of pro-government protesters - and people are saying these are government officials and policemen disguised as civilians - armed with batons, attacking the pro-democracy protesters and dispersing the crowd using violence," our correspondent said.
While it is possible to find Yemenis who say Saleh, 64, is "the best we've had, and after him would be chaos", thousands have hit the streets in protests across the country, including in Aden and Taaz, a southern city, Al Jazeera's Ahelbarra said.
At least 12 people were injured as police shot tear gas to break up protests in the southern city of Taaz, where human rights groups say protesters were also given electric shocks by security officials.
And in Aden, dock workers stormed the offices of the Yemen Gulf of Aden Port Corporation, seizing its chairman, Mohamed Bin Aefan, and other senior corporate figures, they said.
"We have had it with corrupt officials and it's time to tell them to leave," Ali Bin Yehya, a port worker, said.
"What happened in Egypt and Tunisia motivated the workers to demand their rights."
Opposition groups appear to have backtracked on an agreement made on Sunday to meet with Saleh to find a political settlement to the crisis, our correspondent said.
The groups had also aimed to establish a coalition "government of national unity" - after their supporters pointed to the success of Egyptians in overthrowing Hosni Mubarak from power, Ahelbarra said.
The US connection
Saleh has reportedly cancelled a planned trip to the US at this moment of unrest, and will be visiting tribal areas in a bid to convince community leaders not to join the ongoing protests.
Military ties between the US and Saleh's administration have grown stronger in recent months, as the country struggles with the increasing militancy of a secessionist movement in the south, as well as unrest provoked by rising food prices, unemployment reaching 40 per cent - and demands for human rights to be recognised.
The US is shortly to embark on a $75 million project to train Yemen's counterterrorism unit, US officials say.
Saleh became leader of North Yemen in 1978 and has ruled the Republic of Yemen since the north and south merged in 1990.