By Yasmine Saleh

CAIRO (Reuters) - Thousands of supporters of overthrown Islamist president Mohamed Mursi took to the streets in towns and cities across Egypt on Tuesday evening to denounce Egypt's new military-backed rulers - their second show of mass support in four days.

Marking exactly two months since Egypt's first democratically elected leader was ousted by the army after big protests, marchers turned out in cities in the Nile Delta, in Upper Egypt and on the Suez Canal, as well as the capital, Cairo.

The army-led government has launched a furious crackdown on Mursi's Muslim Brotherhood since toppling him on July 3, arresting its top leaders and killing hundreds of his supporters.

But after a brief lull, and despite a heavy security presence, Islamist groups brought thousands onto the streets again after last Friday's prayers. There were sporadic clashes with security forces, notably in Cairo, and at least seven people died.

There were no immediate reports of violence at Tuesday's marches, held under the slogan "The Coup is Terrorism" - a reference to the government's portrayal of its campaign to crush the Brotherhood as a fight against Islamist terrorism.

In Cairo's Nasr City, near the presidential palace, hundreds of Brotherhood supporters waving Brotherhood flags chanted "Revolution, revolution, the revolution will continue!" and "Down, down with military rule!".

Some carried pictures of "martyrs" killed in the government's crackdown, while others stood chanting next to an armoured vehicle, one of many deployed in the capital.

Many of the Brotherhood's leaders including Mursi have already been sent to trial accused of inciting violence, but the movement says it is committed to peaceful protest, and that the accusations are a pretext for the crackdown by a "putschist regime".

A military court sentenced pro-Mursi protesters to long jail terms on Tuesday on charges of attacking soldiers in the city of Suez, a military statement said.

The violence in Suez broke out after security forces on August 14 crushed Cairo protest camps demanding Mursi's reinstatement. More than 600 Brotherhood supporters were killed, along with dozens of policemen, in the dawn operation, which triggered clashes across the country.

The statement said one person had been sentenced to life in prison for the Suez clashes, three people to 15 years in jail, and 45 others to five years.

TELEVISION CLOSURES

TV channels run by the Muslim Brotherhood or sympathetic to it have already fallen victim to the government crackdown.

On Tuesday a Cairo court ordered the closure of the Egyptian news channel belonging to Al Jazeera, the pan-Arab broadcaster financed by Qatar, a supporter of the Brotherhood, along with three other stations run by or sympathetic to the Brotherhood.

Al Jazeera's offices in Cairo have been closed since July 3, when they were raided by security forces hours after Mursi was toppled, although its channels, broadcast from Qatar, can still be seen in Egypt.

Last week, Al Jazeera aired statements from two Brotherhood leaders that included a call to join protests.

On Sunday, three journalists working for Al Jazeera's main, pan-Arab channel were deported from Egypt.

Separately, state-run Nile TV said 15 people had been killed in the Sinai Peninsula by rocket fire, after witnesses said army helicopters had attacked militant strongholds near Sheikh Zuweid, close to the border with Israel and the Gaza Strip.

Security sources said government helicopter strikes had killed at least eight armed men and wounded 15, and had been aimed at stores of arms and explosives.

Militant attacks on security forces in the lawless North Sinai region have grown since Mursi was ousted.

The army has accused Palestinians in Gaza, which is run by Hamas, a Brotherhood offshoot, of supporting the militants.

Mursi's government had made it easier for people and goods to travel between Egypt and Gaza.

But Cairo's new rulers have tightened controls once more, and have been closing smuggling tunnels that the army believes have been used to move weapons and gunmen across the border.

Local residents said on Tuesday that Egyptian security forces had destroyed some 20 houses along the border, apparently suspecting them of being used to hide tunnel entrances or provide cover for other militant activity.

Hamas said it feared Egypt was installing a buffer zone to isolate Gaza. An Egyptian army source confirmed the military had intensified its campaign to close tunnels but said he knew of no instructions to put a buffer zone in place.

Growing insecurity in Sinai worries the United States and others because the region is bounded not only by Israel and Gaza but also by the Suez Canal, a major global shipping artery. Last Saturday, attackers fired at a ship passing through the Canal.

(Writing by Kevin Liffey; editing by Tom Pfeiffer)