Egyptian riot police and protesters observed a truce on Thursday after violence that has killed 39 people in five days and the army said parliamentary elections would start on time next week.

Demonstrations by thousands of Egyptians furious at the slow transfer of power by military leadership to civilian rule have led to violent clashes with police, in scenes similar to the popular uprising that toppled leader Hosni Mubarak in February.

Protesters have vowed not to leave Cairo's central Tahrir Square, which once again has become the cradle of public protest in the most populous Arab country, until army rule ends.

The demonstrations appear to have polarised Egyptians, many of whom worry that unrest will prolong economic stagnation.

In new blows to confidence, the Egyptian pound weakened to more than 6 to the dollar for the first time since January 2005, and Standard & Poor's lowered its rating on Egypt.

The agency cut Egypt's long-term, foreign- and local-currency sovereign credit ratings to B+ from BB-, saying a weak political and economic profile had worsened further.

Egypt's ruling army council said it was doing all it could to prevent more violence. In a statement, it apologised, offered condolences and compensation to families of the dead, and promised a swift investigation into who was behind the unrest.

NO ELECTION DELAY

A ruling council member, General Mamdouh Shaheen, told a news conference the parliamentary vote, whose first stage is due to begin on Monday, would go ahead on time. We will not delay elections. This is the final word, he said.

Another council member, Major-General Mokhtar al-Mullah, took a swipe at the demonstrators. If we look at those in Tahrir, regardless of their number, they do not represent the Egyptian people, but we must respect their opinion, he said.

Mullah said the army hoped to form a new government before Monday to replace Prime Minister Essam Sharaf's cabinet, which resigned during this week's violence without giving a reason.

Demonstrators in Tahrir said the truce had taken hold from midnight. Cranes hauled concrete barriers, later reinforced with barbed wire, across streets leading to the nearby Interior Ministry, flashpoint for much of the recent violence.

Since about midnight or 1 a.m. there were no more clashes. We are standing here to ensure no one goes inside the cordon, said Mohamed Mustafa, 50, among a group barring a street nearby.

They were guarding a barricade made of a broken metal fence, a telephone booth laid on its side and part of a lamp post.

At the other end of the street, littered with shattered glass, lumps of concrete and heaps of rubbish, at least two army armoured personnel carriers blocked the route. Mustafa's group said police were on the front line, and behind them the army.

Lines of Tahrir protesters manned similar barriers to block access to Mohamed Mahmoud Street, scene of repeated fighting.

We have created a space separating us from the police. We are standing here to make sure no one violates it, said Mahmoud Adly, 42, part of a human cordon four ranks deep.

The protests in Cairo and elsewhere pose the gravest challenge to Egypt's army rulers since they took over from Mubarak, overthrown on February 11 after an 18-day uprising.

The thousands who thronged Tahrir Square were undeterred in their determination to rid Egypt of army rule. He goes, we won't, declared a banner referring to the army commander, Field Marshal Mohamed Hussein Tantawi.

The United States and European nations, alarmed at the violence of the past few days, have urged Egypt to proceed with what has been billed as its first free vote in decades.

LACK OF TRUST

The army and the Muslim Brotherhood, which expects to do well in the polls, say it must go ahead, but many protesters do not trust the military to oversee a clean vote. Some scorn the Brotherhood for its focus on gaining seats in parliament.

The military council originally promised to return to barracks within six months, but then set a timetable for elections and drawing up a new constitution that would have left it in power until late next year or early 2013.

Tantawi pledged this week to hold a presidential vote in June that could pave the way for a transfer to civilian rule, but the demonstrators, angered by army attempts to shield itself legally from future civilian control, are unconvinced.

The military council must leave and hand power to civilians. They don't want to leave so that their corruption isn't exposed, said 23-year-old student Ahmed Essam.

Before the truce, protesters had fought running battles with security forces around the Interior Ministry. The bloody chaos there contrasted with normal life in streets nearby.

(Additional reporting by Edmund Blair, Tom Perry and Tamim Elyan; Writing by Alistair Lyon)