Cairo - Thousands of supporters of Egyptian President Mohamed Morsi have gathered here as political violence continued to worsen across the country, with at least two people killed in the northern city of Alexandria.

Clashes broke out on Friday between pro- and anti-government protesters in Alexandria's Sidi Gaber neighbourhood, outside the local headquarters of the Muslim Brotherhood’s Freedom and Justice Party (FJP).

Local health officials said that two people were killed, one of them an American citizen who was stabbed, the other an Egyptian who died from gunshot wounds.

Police said the American was taking photographs of the fighting, but was not believed to be a journalist. Protesters also set fire to the party's offices.

Violent clashes were reported in several other governorates; at least one person was killed early on Friday morning in Sharqiya, in the Nile Delta region, after protesters attacked the FJP offices there.

The rallies in Cairo remained calm, but tensions are clearly running high ahead of nationwide anti-government protests planned for Sunday. On the outskirts of the pro-Morsi rally, rows of men armed with batons and metal rods checked IDs and frisked attendees.

A senior scholar from Al-Azhar, Egypt's highest Sunni religious authority, warned of worsening violence which he blamed on "criminal gangs".

"Vigilance is required to ensure we do not slide into civil war," said Hassan el-Shafei, in remarks carried by state media.

'Here to defend my voice'

Sunday's protests, which organisers hope will draw millions of Egyptians to the streets, will demand that Morsi resign and cede power to a transitional government.

The rally on Friday was intended as a preemptive strike, a chance for organisers to show that Morsi still commands majority support.

"Don't believe that everyone is against the president," said Naeem Ghanem, carrying banners accusing the opposition of working with the United States and Israel. "Ninety percent of the people are with Morsi."

The rally was dubbed "legitimacy is a red line," and demonstrators kept returning to that theme, arguing that the only way to remove the democratically-elected president is through the ballot box. Sunday will mark the end of the first year of Morsi's four-year term.

"I'm here to defend my voice. If you want Morsi to leave, that's fine, but after four years," said Taher Mohamed, manning a stall and selling pro-Morsi gear at the rally.

I'm here to defend my voice. If you want Morsi to leave, that's fine, but after four years. Taher Mohamed, vendor, Protesters railed against popular media figures like satirist Bassem Youssef, accusing them of taking money from Western countries and misrepresenting Morsi's record.

They also mocked a grassroots campaign which claims to have collected signatures from 18 million people demanding Morsi’s resignation, a figure which, if accurate, would overshadow the 13 million votes that brought him to power.

The campaign is called "Tamarod," or "rebellion," and hundreds of their supporters gathered across town in Tahrir Square on Friday, waving red cards to symbolise their demand for Morsi's ouster.

But many of Morsi’s supporters dismiss it as a fraud, a vehicle for former president Hosni Mubarak's regime to regain power, and claim to have met people who signed the petition dozens of times. They have launched their own version, naming it "Tagarod," or "emptiness," and handed out signature forms at Friday's rally.

"But you can only sign it once! We will be watching," one campaigner joked, handing forms to a group of women.

"[The opposition] can oppose [Morsi] within the normal democratic process," said Diaa Agha, a senior member of the FJP's office in Cairo. "But unfortunately they refused all kinds of democracy. They want to overthrow legitimacy by doing illegal acts like the Tamarod campaign."

Demonstrators were largely supporters of the Brotherhood or of other Islamist parties, like the Building and Development Party, the political wing of the once-banned Gamaa al-Islamiyya. But the rally also attracted a number of people who described themselves as political independents.

"We didn't overthrow Mubarak because he was corrupt. We did it because there was no democracy," said Ismail Farid, a retired air force colonel attending the protest, who insisted that he was not an Islamist.

"And now the opposition, a minority in our country, wants thugs to remove our president."