Tens of thousands of Syrians have taken to the streets in the flashpoint city of Homs to rally against President Bashar al-Assad and plead for newly-arrived Arab peace monitors to bear witness to their plight.

About 70,000 protesters marched toward the city centre on Tuesday where security forces fired at them and lobbed teargas, activists said.

The military withdrew some tanks, in what the activists called a ploy to persuade the monitors that the city was calm. Footage on the Internet showed monitors confronted by residents as gunfire crackled around them.

The Arab League observers, who arrived in the country on Monday, want to determine if Assad is keeping his promise to implement a peace plan to end his military crackdown on nine months of popular revolt.

The monitors were due to return Wednesday to Homs where crowds have pleaded for them to visit the most violent neighbourhoods. Activists say tanks ran amok and scores of people have been killed in recent days.

Live broadcasts by Al Jazeera television showed tens of thousands of protesters gathered Tuesday in the Khalidiya district - one of those yet to be visited by monitors - shouting and whistling and waving white flags.

One activist held up a sign to the camera that read: We are afraid when the monitors leave they will kill and bury us.

The observers' visit is the first international intervention on the ground in the country since the uprising began, and protesters hope what they witness will prompt world powers to take more decisive action against Assad.

The Syrian leader says he is fighting an insurgency by armed terrorists, and that most of the violence has been aimed at the security forces. International journalists are mostly barred from Syria, making it difficult to confirm accounts.

The Syrian Observatory for Human Rights, a British-based activist group, said security forces killed 15 people across the country Tuesday, six of them in Homs. An activist network said 34 had been killed Monday.

Some protesters shouted We want international protection in a video posted on YouTube apparently showing an encounter with the monitors Tuesday. Some residents argued and pleaded with them to go further into the Baba Amr quarter, where clashes have been especially fierce.

There was the sound of gunfire after a resident yelled at one monitor to repeat what he had just told his headquarters.

You were telling the head of the mission that you cannot cross to the second street because of the gunfire. Why don't you say it to us? the man shouted, grabbing the unidentified monitor by his jacket.

Gunshots crackled nearby as two monitors and two men wearing orange vests stood amid a crowd of residents, one begging the team to come and see; they are slaughtering us, I swear.

The head of mission said the first visit was very good.

I am returning to Damascus for meetings and I will return tomorrow to Homs, Sudanese General Mustafa Dabi said. The team is staying in Homs. Today was very good and all sides were responsive.

Activist reports just before the monitors arrived Tuesday said up to a dozen tanks were seen leaving Baba Amr and others were being hidden to fashion an impression of relative normality in the city while observers were around.

My house is on the eastern entrance of Baba Amr. I saw at least six tanks leave the neighbourhood at around 8 in the morning (0600 GMT), activist Mohamed Saleh told Reuters by telephone. I do not know if more remain in the area.

Al Jazeera's footage showed thousands of Syrians in the square in Khalidiya, one of four districts where there has been bloodshed as rebels fight security forces using tanks.

They were whistling and shouting and waving flags, playing music over loudspeakers and clapping.

The protesters shouted We have no one but God and Down with the regime. An activist named Tamir told Reuters they planned to hold a sit-in in the square.

We tried to start a march down to the main market but the organisers told us to stop, it's too dangerous. No one dares go down to the main streets. So we will stay in Khalidiya and we will stay here in the square and we will not leave from here.

WASHINGTON CONDEMNS VIOLENCE

The U.S. State Department condemned what spokesman Mark Toner called an escalation of violence before the monitors' deployment.

We have seen horrific pictures of indiscriminate fire, including by heavy tank guns, and heard reports of dozens of deaths, thousands of arrests, as well as beatings of peaceful protestors, Toner said.

The monitors should have unfettered access to protestors and to areas most severely affected by the regime's crackdown. They bear a heavy responsibility in trying to protect Syrian civilians from the depredations of a murderous regime.

If the Syrian regime continues to resist and disregard Arab League efforts, the international community will consider other means to protect Syrian civilians.

Armed insurgency is eclipsing civilian protest in Syria. Many fear a slide to sectarian war between the Sunni Muslim majority, the driving force of the protest movement, and minorities that have mostly stayed loyal to the government, particularly the Alawite sect to which Assad belongs.

Analysts say the Arab League is anxious to avoid civil war. Western powers have shown no desire to intervene militarily in a volatile region of Middle East conflict. The U.N. Security Council is split, with Russia - a major arms supplier to Assad - and China opposed to any hint of military intervention.

Assad's opponents appear divided on aims and tactics. He still has strong support in important areas, including Damascus and the second city Aleppo, and maintains an anti-Israel alliance with Iran.

(Additional reporting by Ayman Samir; Editing by Peter Graff)