Yemen's embattled leader on Monday rejected demands that he step down, calling demonstrations against his regime unacceptable acts of provocation and offering to begin a dialogue with protesters.

However, protests seeking President Ali Abdullah Saleh's ouster did not let up. Tens of thousands rallied in four cities, including the capital of Sanaa. Demonstrators said they would not leave fledgling encampments in central squares until he has left office.

An opposition spokesman rebuffed Saleh's offer of dialogue and an influential group of Muslim clerics called for a national unity government that would lead the country to elections. Saleh has been in power for 32 years.

At least 11 people have been killed since anti-government protests erupted earlier this month, including a youth shot dead Monday, medical officials said. In an attempt to defuse anger over the violence, Saleh told a news conference that he has ordered troops not to fire at anti-government protesters, except in self defense.

The demonstrations in Yemen, the poorest Arab nation, are part of a wave of unrest that have swept across the Arab world in recent weeks and toppled autocratic leaders in Tunisia and Egypt.

Saleh's government was already weak before the protests, facing a southern separatist movement and disaffected tribesmen around the country. Saleh is quietly cooperating with the U.S. in efforts to battle an al-Qaida franchise that has taken root in Yemen, but his government exercises limited control in the tribal areas beyond the capital. The U.S. gives Yemen military aid and training.

Despite Saleh's gestures, protesters are digging in. Several hundred camped overnight in a square in the capital of Sanaa, near the city's university, where their numbers swelled during the day. Many said they would stay until the regime falls.

At one point, Adam al-Hameeri, a soldier, told the crowd that he had joined them becaue he felt the government had neglected the armed forces and pushed them into unnecessary wars.

Several tribal leaders also joined the protest. The sheiks' presence in the square indicated the limited success of Saleh's efforts early in the unrest to win over leaders in the tribes that dominate Yemeni society. One of the leaders, Sheik Khalid Al-Awadi, said he and the others had come to support the protesters' "noble goals."

Similar to the scenes in Tahrir Square in Cairo, the epicenter of the Egyptian uprising, protesters in Sanaa pitched small tents and set up a platform equipped with loudspeakers. Protesters guarded access roads and searched those entering the square.

In the city of Taiz, tens of thousands of protesters rallied in the central square. "We will not leave this place until the downfall of the regime," activist Ahmed Ghilan said.

Thousands also rallied in the port city of Aden. Police fired in the air to disperse demonstrators in one area, while they collected in public squares in others, some planning to camp out.

A large rally was also reported in the city of Saada, near the Saudi border. The area has seen fighting between government forces and Shiite rebels who oppose government control of their area.

Also Monday, the influential Commission of Religious Scholars in Yemen called for the formation of a transition government until new elections can be held. In a statement, the group urged the government to protect peaceful protests and hold accountable those who killed or attacked protesters and journalists.

Saleh claimed Monday that government opponents are a small minority and that those who want to see him leave should compete in elections. The next scheduled vote is in 2013.

"The opposition has raised the ceiling of demands calling now for the regime's down fall. This is unacceptable and those who want power should resort to the polling boxes," he said.

He described the demonstrations as an unacceptable provocation, rejecting what he suggested were attempts at foreign interference. "The arbiter is the (Yemeni) people and not the U.S. embassy, the United States or the EU," he said.

Mohammed al-Sabri, a spokesman for a coalition of opposition groups, rejected Saleh's call for dialogue. "The call is merely an attempt to win time and we have been in this vicious circle for years," he said.

The European Union delegation in Yemen urged authorities in a statement to "immediately halt attacks by security forces and armed pro-government groups on peaceful protesters and