Nabil al-Arabi will visit the Syrian capital with "an initiative" to end the deadlock between the government and protesters, the league said in a statement demanding an end to the bloodshed.
In some of the weekend's heaviest clashes, army defectors who had refused to fire on unarmed protesters reportedly fought loyalist troops in a northeast suburb of the capital.
Dozens of soldiers defected and fled into al-Ghouta, an area of orchards and farmland, after pro-Assad forces shot at a crowd of demonstrators near the Damascus suburb of Harasta to prevent them from marching on the capital, residents said.
Syrian authorities have denied any army defections, though protesters claim growing numbers of rank-and-file soldiers are mutinying against officers loyal to the Assad family.
One resident, who declined to be named, said: "The army has been firing heavy machineguns throughout the night at al-Ghouta and they were being met with response from smaller rifles."
Security forces on Sunday shot dead two and wounded nine others in the northwestern province of Idlib, the Syrian Observatory for Human Rights said.
Abdullah Gul, president of neighbouring Turkey, said he had lost confidence in Damascus's promises to halt the crackdown and deliver reform.
"Today in the world there is no place for authoritarian administrations, one-party rule, closed regimes. Those either will be replaced by force, or the governors of states will take the initiative to administer," Mr Gul warned.
Iran at the weekend warned Mr Assad to heed the "legitimate demands" of his people, but warned Nato would become bogged down in a quagmire if it interfered.
Ali Akbar Salehi, Tehran's foreign minister, said: "Syria is the front-runner in Middle Eastern resistance (to Israel) and Nato cannot intimidate this country with an attack.
"If, God forbid, such a thing happened, Nato would drown in a quagmire from which it would never be able to escape ...
"If the West should want to follow the same course as they have done in Iraq and Afghanistan they would not realise the desired result."
William Hague, British Foreign Secretary, ruled out a Libya-style Nato military campaign in Syria.
He claimed the success of Libyan rebels in taking Tripoli "vindicated" Britain's policy of military action, but said there was no consensus for action in Syria.
It was unclear when the Arab League delegation would reach Damascus and details of the peace plan were not disclosed.
Russian diplomats were also preparing to send their own delegation with a competing initiative, Moscow said.