CAIRO (AP) -- Thousands flocked to Cairo's central Tahrir square on Tuesday for a protest against Egypt's president in a significant test of whether the opposition can rally the street behind it in a confrontation aimed at forcing the Islamist leader to rescind decrees that granted him near absolute powers. Waving Egypt's red, white and black flags and chanting slogans against President Mohammed Morsi and his Muslim Brotherhood, the protesters joined several hundreds who have been camping out at the square since Friday demanding the decrees be revoked. Even as the crowds swelled, clashes erupted nearby between several hundred young protesters throwing stones and police firing tear gas on a street off Tahrir leading to the U.S. Embassy. Clashes have been taking place at the site for several days fueled by anger over police abuses, separately from the crisis over Morsi. The president's declaration last week of new powers for himself has energized and - to a degree unified - the mostly liberal and secular opposition after months of divisions and uncertainty while Islamists from the Muslim Brotherhood and other groups rose to dominate the political landscape. The turnout for Tuesday's protest call is key to whether the opposition can keep a movement going against Morsi. While the edicts last week sparked the protests, they have also been fueled by anger over what critics see as the Brotherhood's monopolizing of power after its election victories the past year for parliament and the presidency. "We want to change this whole setting. The Brotherhood hijacked the revolution," said Rafat Magdi, an engineer who was among a crowd of around 2,000 marching from the Cairo district of Shubra to Tahrir to join the rally. "People woke up by his mistakes, and any new elections they will get no votes." Morsi's decrees, issued Thursday, placed him above any kind of oversight, including that of the courts, until a new constitution is adopted and parliamentary elections are held - a timeline that stretches to mid-2013. The opposition says the decrees give Morsi near dictatorial powers by neutralizing the judiciary at a time when he already holds executive and legislative powers. Leading judges have also denounced the measures. Morsi says the decrees are necessary to protect the "revolution" and the nation's transition to democratic rule. His declaration made all his decisions immune to judicial review and banned the courts from dissolving the upper house of parliament and an assembly writing the new constitution, both of which are dominated by Islamists. The decree also gave Morsi sweeping authority to stop any "threats" to the revolution. By early afternoon, nearly 20,000 people were at Tahrir, birthplace of the 18-day popular uprising that toppled President Hosni Mubarak's authoritarian regime nearly two years ago. Security was tightened across Cairo, with police checking ID papers and searching cars coming into the center, but there was no sign of protesters being stopped from reaching Tahrir. A chaotic city of some 18 million people, Cairo's traffic was uncharacteristically light on Tuesday, with many businesses and government offices closing shop early in anticipation of possible unrest. A new banner in the square proclaimed, "The Brotherhood stole the country" and one protester, Mahmoud Youssef, said: "We are here to bring down the constitutional declaration issued by Morsi." Many chanted the iconic slogans "the people want to bring down the regime." Several thousand lawyers meanwhile gathered outside their union building in downtown Cairo ahead of their march to Tahrir to join protesters there. "Leave, leave," they chanted, addressing Morsi, who narrowly won elections in June to become the country's first freely elected civilian president.