About 1,000,000 people have gathered for the planned "march of a million" in the Egyptian capital, calling for Hosni Mubarak, the embattled Egyptian president, to step down.
Meanwhile, one of Egypt's oldest parties, Wafd, announced on Tuesday that a number of opposition groups have agreed to form "a national front" to deal with the volatile situation there. In a statement, Wafd said that president Mubarak "has lost legitimacy".
Also on Tuesday, the Muslim Brotherhood, an officially banned but tolerated movement, said it will not negotiate with president Mubarak or his government.
Earlier, some opposition parties have called for Mubarak to delegate responsibilities to newly appointed vice-president Omar Suleiman, who they are prepared to negotiate with.
Thousands of demonstrators began gathering from early on Tuesday morning in Cairo's Tahrir Square, which has been the focal point of protests in the capital and served as the meeting area for the march to begin on the eighth day of an uprising that has so far claimed more than 125 lives.
Another protest in the Mediterranean port city of Alexandria attracted tens of thousands of protesters, as national train services were cancelled in an apparent bid to stymie protests. Protests were also reported in the city of Suez.
Protest organisers have also called for an indefinite strike to be observed across the country.
Our producer in Egypt reports on the latest developments
Soldiers at Tahrir Square have formed a human chain around protesters, and are checking people as they enter for weapons. Tanks have been positioned near the square, and officers have been checking identity papers.
The army has also blocked all major roads in the city, and tens of thousands of protesters are being held at the Kasr al-Nile bridge. They were on their way to the main protest at Tahrir Square.
Al Jazeera correspondents have described a "festival-like" and "communal" atmosphere at the protest, with protesters from all walks of life represented.
"It is peaceful, people power that has united here in the heart of Egypt's historic square," reported one correspondent.
An Al Jazeera correspondent in Cairo said that there were reports that "thugs in certain parts of the city have been trying to stop people from driving into Cairo".
She said that "increasingly large pockets of pro-government protests" are also taking place at various locations in the city. There are fears that if the two sets of protesters meet, a violent clash could erupt.
Gigi Ibrahim, a political activist who planned to attend the rally, told Al Jazeera the protesters will not be satisfied until Mubarak steps down.
"I think today there will be great numbers on the street ... every day there are more numbers on the street than the day before. I think the protests are gaining momentum. The people ... will literally not leave until Mubarak steps down," she said.
In an attempt to discourage people from the protests, Egyptian state television has asked people to stay at home, warning of possible violence.
An Al Jazeera online producer in Cairo said that if today's protest does not go as planned, similar protests could be planned for Friday.
Protests are also taking place in the cities of Mansoura, Damnhour, Arish, Tanta and El-Mahalla El-Kubra.
The new protests come as the police have returned to the streets.
But while the police's posture to be adopted in the face of the strike and marches remains unknown, the Egyptian army stated clearly on Monday that it would not stop protests
Faced with the prospect of massive numbers trying to converge on the capital, Egyptian authorities stopped all train traffic with immediate effect on Monday afternoon, and the state-owned national carrier EgyptAir said it was cancelling all international and domestic flights during curfew hours (3.00pm to 8.00am local time).
In a statement on Monday, the army said "freedom of expression" was guaranteed to all citizens using peaceful means.
"To the great people of Egypt, your armed forces, acknowledging the legitimate rights of the people," stress that "they have not and will not use force against the Egyptian people," said the statement.
It was the first such explicit confirmation by the army that it would not fire at demonstrators who have taken to the streets of Egypt and comes a day before Tuesday's "march of millions".
Our producer in Egypt reports on the latest developments in Tahrir Square
It urged people not to resort to acts of sabotage that violate security and destroy public and private property. It warned that it would not allow outlaws to loot, attack and "terrorise citizens".
The call for the "million-man-march" from the so-called April 6 movement has come as Mubarak swore in a new cabinet on Monday, in an attempt to defuse ongoing demonstrations across the country.
Panic and chaos
On Tuesday, even as Egypt continued to face economic turmoil as a result of protests, the International Monetary Fund said it was ready to put in a place an economic rebuilding policy for the country.
"The IMF is ready to help in defining the kind of economic policy that could be put in place," IMF chief Dominique Strauss-Kahn said.
Meanwhile, chaos has been reported at Cairo's international airport, where thousands of foreigners are attempting to be evacuated by their home countries.
Our correspondent reported on Tuesday that about 1,000 US citizens have been evacuated to Cyprus or Turkey, from where they are expected to make their own way home.
She also said that China is sending two additional planes to evacuate its citizens.