The activists also posted a brief clip on YouTube, showing the US envoy Robert Ford and his French counterpart Eric Chevallier sitting on chairs at a large ceremony.
Matar, a key player in organising protests against President Bashar al-Assad's regime, died in detention after being tortured, according to the international watchdog Human Rights Watch (HRW).
Activists said that the Japanese and Danish ambassadors also attended the ceremony, and that immediately after diplomats departed, security forces attacked the ceremony, launching tear gas and firing in the air to disperse the gathering.
Meanwhile, demonstrators burned Russian flags in the flashpoint protest hubs of Homs in the centre and Daraa in the south in protest at Moscow's support for President Bashar al-Assad, activists said.
Turkish Prime Minister Recep Tayyip Erdogan warned Syria could plunge into "civil war," as he began in Egypt a tour of Arab countries where uprisings have ousted autocratic leaders.
He expressed frustration with Assad's regime, with which he had built up close ties, for failing to "listen to the voice of the people."
Syrian Protesters have been demanding democracy in almost daily demonstrations for six months, with the United Nations saying 2,600 people have been killed in the regime's crackdown.
In Cairo, the Arab League called for "immediate change" in Syria.
"There must be an immediate change that leads to an end to the bloodshed and protection of the Syrian people from more violence and killings," said a statement read out by Qatar's Foreign Minister Sheikh Hamad bin Jassim al-Thani, the chair of the meeting.
"We cannot accept this killing machine. We cannot allow people to be killed this way," Sheikh Hamad said at a press conference after the meeting.
"Do not support the killers," activists urged Russia in a message announcing Tuesday's action posted on The Syrian Revolution 2011, a Facebook page that has been a driving force behind the protest movement.
"We express our anger towards Russia and the Russian government. The regime will disappear but the people will live," the activists added.
Moscow has blocked Western-led efforts at the UN Security Council to impose sanctions against the Syrian regime and is promoting a rival draft resolution that simply calls on the government and the opposition to open direct talks.
President Dmitry Medvedev defended the Russian position on Monday, saying "this resolution must be strict, but it must not lead to the automatic application of sanctions."
On Tuesday, police and troops again deployed in force, carrying out search and arrest operations in a string of towns, activists said.
"Those arrested were severely beaten and abused. Their houses were ransacked," one activist said.
At least four people were killed Tuesday in Deir Ezzor province in the northeast and in Hama, while five were wounded when troops went house-to-house in Houla in Homs province, the Syrian Observatory for Human Rights told AFP in Cyprus.
Also in Homs, two people were reported dead, one of whom was kidnapped four days ago and whose corpse was handed to the family and a second succumbing to injuries suffered during security operations Saturday, the Observatory said.
In addition, security forces arrested at least 34 people in the town of Zabadani west of Damascus, more than 160 in Idlib province near the Turkish border, and dozens more in Daraa, in towns around the capital, and in the Mediterranean coastal towns of Latakia and Banias, activist groups said.
UN High Commissioner for Human Rights Navi Pillay said that, as of Monday, a total of 2,600 people had been killed in the Syrian government's crackdown.
But senior Assad aide Bouthaina Shaaban said on a visit to Moscow that 1,400 people had died since the demonstrations erupted in mid-March -- half of them security force personnel and half of them "rebels".
Damascus has consistently maintained that the protests are the work of armed groups, rejecting the reports of Western embassies and human rights groups that the great majority of those killed have been unarmed civilians.
In Brussels, diplomats told AFP Tuesday that the EU is SET to adopt fresh sanctions against Syria that may include a ban on oil investment and on delivering cash notes printed in Europe to its central bank.
After talks in Beijing on Tuesday, French Foreign Minister Alain Juppe expressed disappointment that he had made little headway in persuading his Chinese counterpart Yang Jiechi to abandon its opposition to a sanctions resolution.
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