He added, "We want people in Syria to see how we are supporting their cause here in Egypt."
Most of the protesters were Egyptian, Tarek Shalaby, an Egyptian activist and web designer at the scene.
Alla Mahmoud, a spokesman from the Interior Ministry, told CNN that 15 protesters were arrested. Some suffered minor injuries, from excessive inhalation of tear gas to bruises.
Meanwhile, U.S. President Obama called Russian President Vladimir Putin on Wednesday to discuss the Syrian situation, the White House said. "They noted the differences our governments have had on Syria, but agreed to have their teams continue to work toward a solution," it said.
A planned Security Council vote on a draft resolution on Syria was delayed at the request of Kofi Annan, joint envoy to Syria for the United Nations and the Arab League, until 10 a.m. Thursday, diplomats said.
Annan, who met Tuesday with Putin in Moscow, earlier this year put forward a peace plan that has failed to stop the violence.
Western countries are pushing for a resolution that threatens sanctions against al-Assad's regime if government forces don't stop attacks. That draft also calls for renewing the 300-member U.N. observer mission for 45 days.
The observers' work has been suspended because of violence.
Throughout Syria's 16-month crisis, Russia has opposed any international effort that would blame, punish or change the leadership of the Syrian government. Russia -- along with China -- has vetoed two previous draft resolutions in the U.N. Security Council, leading to accusations that Russia is protecting the Syrian regime.
Russia, meanwhile, has put forth its own draft, which "strongly urges all parties in Syria to cease immediately all armed violence in all its forms." The Russian draft also calls for renewing the U.N. observer mission for three months.
At the United Nations, German Ambassador Peter Wittig said Wednesday's events in Damascus "underline as clearly as never before that the Security Council now has to act. It has to send out a strong signal to the Syrian regime to stop the use of heavy weapons. We want to give the Annan plan, which we all support in the council, some teeth -- and that's why we should adopt a resolution with the sanctions threat."
In Amman, Jordan, King Abdullah said the situation in Syria is nearing an all-out civil war. "In other words, it's getting very, very messy," he said. "When you get full-out civil war, there is no coming back from the abyss."
The attack represents "a massive psychological blow to the regime" and will accelerate al-Assad's "demise," predicted Anthony Skinner, an analyst with Maplecroft, a think tank that provides risk assessments on global business.
It could suggest that "the regime itself is crumbling," said Rime Allaf, an analyst with Chatham House, a think tank focusing on international affairs.
Events in Syria show "a real escalation in fighting," said U.S. Defense Secretary Leon Panetta.
It "tells us that this is a situation that is rapidly spinning out of control, and for that reason it's extremely important that the international community, working with other countries that have concerns in that area, have to bring maximum pressure on Assad to do what's right, and to step down and to allow for that peaceful transition," Panetta said.
The U.S. government announced Wednesday a new round of sanctions against members of the Syrian government.
Syrian Information Minister Omran al-Zubi, speaking on state TV, vowed that those behind the attack will be held accountable.
He insisted that those trying to divide the army are failing. "This army has not been divided," he said.
But increasing numbers of officials in the Syrian military have defected in recent days. Two brigadier generals fled overnight to Turkey, bringing the number of Syrian generals in Turkey to 20, a Turkish Foreign Ministry official said.