Tamo founded the liberal Kurdish Future Party, which considers Kurds an integral part of Syria, and had been recently released after three and a half years in prison.
His killing also sparked indignation abroad.
European Union foreign policy chief Catherine Ashton firmly condemned the murder "in the strongest terms," a statement issued on her behalf said.
It added that Tamo's death "follows other targeted assassinations in the past days, which are totally unacceptable.
"These appalling crimes further add to the EU's grave concern over the situation in Syria. All those responsible for and complicit in these crimes must be held accountable."
In Beirut, 50 activists gathered outside the Syrian embassy to demand Assad's departure, and in Vienna 11 people were arrested overnight for invading Damascus's embassy and demonstrating on a balcony.
Police in London arrested seven protesters outside the Syrian embassy, including three men who climbed onto the roof and waved the Kurdish flag.
The United States on Friday charged that Assad's regime was escalating its tactics against the opposition with bold attacks on its leaders, and France said it was "shocked" by the news of Tamo's murder.
Syria closed one of its border gates with Turkey and barred Turks from entering Syria following the Qamishli clashes, Anatolia news agency reported.
Turks were not allowed to enter Syria from the border gate in Nusaybin city in southeast Turkey, just a few kilometres (miles) from Qamishli, local governor Murat Girgin told Anatolia.
The Local Coordination Committees inside Syria, meanwhile, accused Assad's regime of trying to "physically eliminate" opposition figures and "taking advantage of the laxity of the international community."
The official SANA news agency reported "the assassination" of Tamo and said he was killed "by gunmen in a black car who fired at his car."
Elsewhere Saturday, cell phone and landline communications were cut in Qusayr near the central city of Homs, raising fears of a military operation after checkpoints were SET up and tanks deployed, the Observatory said.
It reported one killed by security forces in Homs and another three in Hama, and said that the bodies of two people who had been arrested were returned to their families.
In addition to the two killed at the Tamo funeral, two people were killed and 10 others wounded when security forces opened fire at a funeral for three people shot dead in Douma near Damascus on Friday, the rights group added.
The UN Committee on the Rights of the Child has reported that at least 187 children have died in Syria among the estimated total of more than 2,900 people killed since Syria launched its brutal crackdown on dissent in mid-March.
In a rare show of diplomatic support, the foreign ministers of Venezuela and Cuba are to lead a delegation of leftist Latin American states to meet Assad on Sunday, Venezuelan Foreign Minister Nicolas Maduro said.
Syrian dissidents, meanwhile, were in Cairo where the Arab League has its headquarters to lobby for recognition of the SNC.
Yasser Najjar, a member of the delegation, told official Egyptian news agency MENA the delegation sought support for the recognition of the SNC, after which it would meet to elect a leadership.
Another 90 members of Syria's opposition gathered in Stockholm for a strategy meeting.
Russia has said it will host opposition figures next Tuesday after President Dmitry Medvedev unexpectedly piled pressure on Damascus to implement reforms announced by Assad.
"If the Syrian leadership is unable to undertake these reforms, it will have to go. But this is something that has to be decided not by NATO or individual European countries but by the people and leadership of Syria," he said.