CAIRO - Thousands of students from Egypt's al-Azhar University staged a third day of protests on Monday, security sources said, in one of the boldest challenges to the army since it toppled Islamist President Mohamed Morsi in July.The demonstrations demanding Morsi's reinstatement are a delicate matter for the authorities because the administration at al-Azhar, the ancient seat of Sunni Muslim learning, has historically toed the government line.In another sign of Egypt's struggle to impose stability since Morsi's ouster, the prime minister on Monday threatened tough measures against anyone who attempts to divide Egypt after gunmen killed four people in an attack on a church in Cairo.CONTROVERSIAL ANTI-PROTEST LAW"As well as placing restrictions on the right to freedom of assembly the proposed law would give security forces a free rein to use excessive and lethal force against demonstrators - including supporters of deposed President Mohamed Morsi," said Hassiba Hadj Sahraoui, Amnesty International's Deputy Director for the Middle East and North Africa.After toppling Mursi in July, army chief General Abdel Fattah al-Sisi appeared on state television to announce a political road-map that would lead to free and fair elections.In an assertion of sectarian harmony, Sisi was flanked by a senior Muslim cleric and the Coptic Christian pope, but a bloody security crackdown on Morsi supporters on Aug. 14 was followed by Egypt's worst attacks on churches and Christian property in years, most of them occurring outside Cairo. The authorities have accused the Brotherhood of being behind the violence, a charge it rejects.Minority Coptic Christians fear an Islamist backlash against their community which is widely seen as backing Morsi's ouster.On Sunday, gunmen on a motorcycle fired on wedding guests outside a Coptic church in a Cairo suburb. An eight-year-old child was among the four people killed. Six were wounded."Christians are the target," said one church official, who asked not to be named. "Low security around churches has always been an issue...Still we know we are a target for Islamists."Prime Minister Hazem el-Beblawi said in a statement that such "heinous acts" would not succeed in dividing Egyptians and that the government would watch out for attempts to sow discord.Coptic Christians make up 10 percent of Egypt's 85 million people, and have generally coexisted peacefully with majority Sunni Muslims for centuries, despite bouts of sectarian tension.The protests at al-Azhar campuses in Cairo and other cities are smaller than previous rallies against the army-backed government. Security sources said a total about 4,000 students were involved, of whom 44 had been arrested.The unrest suggests Morsi supporters may have shifted tactics, focusing on sensitive sites rather than huge street protests which often lead to strong action by security forces.Some clerics, officials and professors at al-Azhar are known to be supporters of Morsi's Muslim Brotherhood.It is not clear whether the protests reflect serious splits between them and their opponents at al-Azhar, or whether a group of students is simply trying to pressure the government.Authorities have been cracking down hard on the Brotherhood, which has won every vote since a popular uprising toppled autocrat Hosni Mubarak in 2011, but is now outlawed again. Security forces have killed hundreds of people in protests. Brotherhood leaders, including Morsi, Egypt's first freely elected president, have been jailed on charges of inciting violence - allegations they deny.The student demonstrations erupted as a debate grows over a draft law that would severely restrict protests.Human rights group say the law would only bring more bloodshed to Egypt, a US ally which lies at the heart of the Middle East and controls the Suez Canal, a global trade route.