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.
In the Mediterranean city of Alexandria, some 3,000 anti-Morsi protesters gathered outside the main court at the center of the ancient city.
Morsi's supporters canceled a massive rally they had planned for Tuesday, citing the need to "defuse tension" after a series of clashes between the two camps since the decrees were issued Thursday.
But a Brotherhood spokesman said demonstrations supporting the president could go ahead outside the capital and that supporters would form human chains in some provinces to protect Brotherhood offices. Morsi's supporters say more than a dozen of their offices have been ransacked or set ablaze since Friday. Some 5,000 demonstrated in the southern city of Assiut in support of Morsi's decrees, according to witnesses there.
On Monday, Morsi met with the nation's top judges and tried to win their acceptance of his decrees. But the move was dismissed by many in the opposition and the judiciary as providing no real concessions.
Presidential spokesman Yasser Ali, said Morsi told the judges that he acted within his rights as the nation's sole source of legislation, assuring them that the decrees were temporary and did not in any way infringe on the judiciary. He underlined repeatedly that the president had no plans to change or amend his decrees.
According to a presidential statement late Monday, Morsi told the judges that his decree meant that any decisions he makes on "issues of sovereignty" are immune from judicial review.
The vaguely worded statement did not define those issues, but they were widely interpreted to cover declaration of war, imposition of martial law, breaking diplomatic relations with a foreign nation or dismissing a Cabinet. Morsi's original edict, however, explicitly gives immunity to all his decisions and there was no sign it had been changed.
Monday's presidential statement Monday did not touch on the immunity that Morsi gave the constitutional assembly or the upper chamber of parliament, known as the Shura Council. It also did not affect the edict that the president can take any measures he sees as necessary to stop threats to the revolution, stability or public institutions. Many see that edict as granting Morsi unlimited emergency powers.
The Shura Council does not have lawmaking authorities but, in the absence of the more powerful lower chamber, the People's Assembly, it is the only popularly elected, national body where the Brotherhood and other Islamists have a majority. The People's Assembly was dissolved by a court ruling in June.
Rights lawyers and activists, however, dismissed Morsi's assurances as an attempt to defuse the crisis without offering concrete concessions.
One of the lawyers, Ahmed Ragheb, described the presidential statement and Ali's comments as "playing with words."
"This is not what Egyptians are objecting to and protesting about," he said. "If the president wanted to resolve the crisis, there should be an amendment to his constitutional declaration."
On Tuesday, the influential Judges' Club, a sort of union led by an outspoken Morsi critic, vowed in a statement to escalate its resistance to the decrees. The statement was carried by Egypt's official MENA news agency, which also reported that judges and prosecutors continued their strike for the third day Tuesday, leaving many courtrooms empty across the nation.
(Copyright 2012 by The Associated Press. All Rights Reserved.)