Hundreds of thousands of anti-government protesters marched Sunday in Yemen's capital Sanaa to press President Ali Abdullah Saleh to resign amid escalated tension as both the government forces and the defected army are deploying more troops in major cities to cope with any possible clashes.

At least four protesters were injured by gunshot of police when the latter tried to force hundreds of thousands of demonstrators to return to their epicenter square in Sanaa on Sunday, eyewitnesses said.

Two armored vehicle of the defected army, which sided with the protesters and vowed to protect them, then rushed to the scene near the compound of Vice President Abd-Rabbu Mansour Hadi and fired to the air, according to eyewitnesses.

"The injured were taken to the field hospital at the Changing Square outside Sanaa University," eyewitness named Adel al-Yazidy told Xinhua.

Before they were forced to withdraw, the protesters took to a street leading to Saleh's presidential palace, chanting "Peaceful, peaceful revolt... No for civil war."

Similar anti-government marches were simultaneously staged in other major provinces, including Taiz, Al-Hodayda, Hajja, Dhamar, Ibb, Amran and Saada, according to several local witnesses.

The move came as part of escalating step announced by the opposition's National Committee for Peaceful Revolution Forces in order to exert pressure on Saleh to resign.

Meanwhile, the Interior Ministry on Sunday held the leadership of the opposition coalition Joint Meeting Parties (JMP) responsible for riot against government facilities in a statement obtained by Xinhua.

The ministry directed security forces to tighten all entrances leading to Sanaa as a precautionary measure to "prevent any smuggling of pro-protest armed tribesmen."

The government forces were on high alert as defected army began to deploy forces to the streets of Sanaa and other major cities since early Saturday.

Sanaa has been hit by blackouts for tens of hours since Saturday evening, and almost all petrol stations were shut down after oil tankers were intercepted at the capital's entrances and were prevented from supplying fuel to the stations, local residents and drivers said.

Late on Saturday, Mohammed Basindwah, chairman of the National Council, vowed to implement escalated protests against the government and warned that the opposition would not deal with the initiative brokered by the Gulf Cooperation Council (GCC) unless Saleh resigns first.

In a meeting with members of the National Council, Basindwah was also cited by opposition media as stipulating that "the GCC's condition of granting Saleh and his family immunity from prosecution should be excluded."

Yemen has been gripped by a political crisis since late January when protests demanding the end of Saleh's 33-year rule erupted.

Saleh, who is still in Saudi Arabia for rehabilitation from injuries he sustained in a shelling attack on his presidential compound on June 3, said in an audio speech aired by the state TV on Monday that he was willing to sign the GCC deal to prepare for holding elections for a new president.