Announcing the deadline in Doha on Saturday, the prime minister of Qatar urged Syria to accept the observers.
"During the meeting we contacted Damascus... and we asked them to come tomorrow (to Doha) to sign" the protocol on sending observers to Damascus, Sheikh Hamad bin Jassim al-Thani said.
"We are waiting for a reply," he said. "As Arabs we fear that if the situation continues things will get out of Arab control."
He was speaking after a meeting to discuss a set of Arab League sanctions against Damascus over its bloody crackdown on more than eight months of anti-regime protests.
The meeting issued a list of 19 Syrian officials banned from traveling to Arab countries and whose assets are being frozen by those countries.
Among those named are Assad's brother, Maher al-Assad, his cousin and telecom magnate Rami Makhluf, as well as military and intelligence figures.
The meeting also decided to cut by half all Arab flights to and from Syria effective December 15, including those of the national carrier Syrian Air, a statement said.
The Arab League last Sunday approved sweeping sanctions against Assad's government over the crackdown -- the first time that the bloc has enforced punitive measures of such magnitude on a member.
The measures included an immediate ban on transactions with Damascus and its central bank and a freeze on Syrian government assets in Arab countries.
The vote on sanctions came after Damascus defied an earlier ultimatum to accept observers under an Arab League peace plan.
Syria's Foreign Minister Walid Muallem said last month that the wording of the text to send observers undermined the country's sovereignty because it "totally ignores the Syrian state, even coordination with the Syrian state."
On the ground, 11 civilians were among 23 people killed in Syria on Saturday, the Britain-based Syrian Observatory for Human Rights said.
Most of the deaths occurred in the northwestern province of Idlib, a focal point of anti-regime protests raging since March, where rebels clashed with loyalist troops near the provincial headquarters, leaving 15 people dead.
Three civilians caught in the crossfire were killed, the Observatory said. The other deaths were seven pro-regime soldiers, security service agents and five rebel troops.
The unprecedented movement against Assad's regime has been spearheaded by peaceful demonstrators, but in recent months army deserters have formed a rebel Free Syrian Army which has inflicted growing losses on regular forces.
UN High Commissioner for Human Rights Navi Pillay said more than 4,000 people have been killed in the crackdown and tens of thousands arrested.
At least 12,400 people are reported to have fled the country.
In Geneva on Friday, an emergency meeting of the Human Rights Council passed a resolution "strongly condemning the continued widespread, systematic and gross violations of human rights and fundamental freedoms by the Syrian authorities."
But the Syrian foreign ministry rejected the resolution as "unjust" and said it was "prepared in advance by parties hostile to Syria."
The violence in Syria topped talks Friday between the US vice president and Turkish President Abdullah Gul in Ankara.
"Assad and his regime are the source of instability in Syria now and pose the greatest danger to fanning flames of sectarian conflict not only in Syria but beyond," Biden told Gul Saturday.
The United States stands with Turkey "in growing out of patience and calling for President Assad to step aside," Biden also said.
Meanwhile the leader of the main opposition Syrian National Council (SNC) appealed Saturday for a UN Security Council resolution to end the regime's deadly crackdown on civilians, while ruling out military intervention.
"What we will seek from the Security Council, above all, are mechanisms for the protection of the innocent civilians," Burhan Ghaliun told journalists on the sidelines of a transitional democracy forum in Sofia.