* Referendum important step to elections
* Protesters defy new law restricting demonstrations
* Police fire teargas in Cairo
Egypt will hold a referendum on an amended constitution in December, the group drafting it said on Tuesday, an important step in an army-backed roadmap meant to lead to elections.
Hours before the timing of the referendum was announced, protesters took to the streets in defiance of a law passed on Sunday requiring police approval for gatherings of more than 10 people. Police detained 28 people, the Interior Ministry said.
Egypt's democratic credentials have been called into question since the military toppled the country's first freely elected president, Islamist Mohamed Mursi, in July, following mass protests against his rule.
A committee of 50 members, with few Islamists, began work in September on amending the constitution that was approved in a referendum last year after being drafted by an Islamist-dominated assembly.
"The referendum will be held before the end of (December)," Mohamed Salmawy, spokesman of the constituent assembly, said. That contradicts comments made by Prime Minister Hazem el-Beblawi who said on Sunday the referendum would be held in the second half of January.
The new constitution will guarantee the right to protest and ensure that demonstrations can be held if protesters notify authorities, rather than wait to be granted permission, Salmawy said, in an apparent effort to ease tension over a new law restricting demonstrations.
Army chief General Abdel Fattah al-Sisi, who ousted Mursi, has promised the roadmap will lead to free and fair elections.
But the plan has not stabilised Egypt, where protests and attacks by Islamist militants based in the unruly Sinai Peninsula have hammered investment and tourism.
Egypt has stumbled on its path to democracy since a popular uprising ousted autocrat Hosni Mubarak in 2011, with the army ousting Mursi and security forces mounting one of the fiercest crackdowns on Islamists in decades.
Hundreds have been killed and the Brotherhood's leadership has been arrested.
Khaled Dawoud, a prominent liberal political figure, criticised the government.
"They are making more enemies than friends especially among the young revolutionary Egyptian people who have been in the streets for the past three years," he said.
Underscoring diffferences over the new law, ten members of the body have suspended their work in protest against the detentions, MENA reported. Beblawi promised to follow the results of the prosecutor's investigations into the detentions, according to an emailed statement from the cabinet.
Human rights groups have condemned the law as a major blow to freedom in Egypt, the most populous Arab state and a U.S. ally that has experienced near relentless upheaval since autocratic ruler Hosni Mubarak was toppled by a revolt in 2011.
"(The) new protest law gives security forces free rein," Amnesty International said.
Skirmishes broke out between security forces and protesters in downtown Cairo and police fired teargas and water cannon to disperse the demonstrations. They were marking the death of a liberal activist killed in clashes with police two years ago and expressed anger against the protest law.
"DOWN WITH MILITARY RULE"
Hundreds assembled at the Press Syndicate and parliament. "Down, down with military rule," they chanted.
In Cairo, female students at Al-Azhar University for Islamic learning, which follows the government line, stormed into a dean's office and destroyed her desk.
The United States, which has partially frozen aid to Egypt, on Monday expressed concern over the new law and said it agreed with groups that argued it did not meet international standards and hampered the country's move toward democracy.
A security official said Tuesday's crowd had not obtained permission to protest and had ignored warnings to disperse.
The army-backed government has said it is not against peaceful protests but wants to restore order in the streets. It has also complained that protests often disrupt traffic. Some Egyptians cheered police as they broke up protests on Tuesday.
"We are implementing the new protest law that requires protesters to seek permission from the Interior Ministry three days before the protest," a police official said.
The protest law will further squeeze members of the Muslim Brotherhood, who have said they will continue demonstrating against what they say is a military coup.