The U.S. has been tightening security at diplomatic installations across the world, as authorities brace for more violent demonstrations in the Middle East against an American made anti-Islam film.

Crowds are expected to gather after Friday prayers. In the early morning hours, Egyptian police used tear gas to disperse protesters near the U.S. Embassy in Cairo for the fourth straight day.

Demonstrations that began earlier this week in Egypt and Libya spread to other areas, including Yemen, Iran, Iraq and Bangladesh on Thursday.

Meanwhile, Libyan officials have arrested four people in connection with this week's assault on the U.S. consulate in Benghazi that killed U.S. Ambassador Christopher Stevens and three of his staff. The officials did not provide details.

American intelligence agencies are examining the alleged involvement of pro-al-Qaida militants. But they say they do not have solid evidence.

U.S. Secretary of State Hillary Clinton condemned the amateur U.S.-made movie that mocks the Islamic Prophet Muhammad. She called it “disgusting” and “reprehensible.”

Protests in Yemen Thursday left four people dead. Yemeni President Abed Rabbo Mansour Hadi immediately condemned the violence and promised to protect U.S. citizens in Yemen. The White House says U.S. President Barack Obama telephoned Mr. Hadi to thank him for his swift response.

Egyptian President Mohamed Morsi also condemned both the film and the violence Thursday. He promised to protect foreigners in Egypt.

The United States has sent two Navy destroyers, a Marine Corps anti-terrorist security team and agents from the Federal Bureau of Investigation to Libya to protect Americans.