Protesters clashed Friday with security forces in the Libyan capital of Tripoli and fought for control of key eastern cities in the most serious challenge to dictator Moammar Gadhafi’s 42 years in power, according to witnesses, online posts and news reports.

The Libyan bloodshed appeared to be the worst in the Middle East on a day that also saw troops fire on pro-democracy protesters in the Persian Gulf sheikdom of Bahrain and confrontations pitting protesters against security forces and loyalists in Yemen.

Across the Gulf of Aden from Yemen, riot police used tear gas and batons to break up a protest by several thousand anti-government demonstrators in Djibouti, a city-state of 750,000 people in the Horn of Africa that hosts the only U.S. military base in Africa.

“The Arab world is experiencing a domino effect,” said Ghada el Sherif, 40, a demonstrator in Cairo.

In Libya, there were unconfirmed reports of many deaths and injuries on the third day of clashes between security forces and regime loyalists on one side and protesters inspired by the popular uprisings that ousted former rulers of Egypt and Tunisia on the other.

“We are hearing now the gunfire. We are hearing explosions from everywhere,” Braikah, a doctor, said by telephone from Benghazi, Libya’s second-largest city. She asked that her last name not be used for her own security. More than 30 people have been killed in Benghazi alone, and hundreds injured, she said.

In Yemen, government supporters, angered by an eighth day of demonstrations demanding that President Ali Abdullah Saleh step down, invaded the opposition rallies and setting off riots in several cities.

Three people have been killed in the protests ignited by widespread poverty and alleged government corruption, including an anti-government demonstrator killed Friday when a hand grenade was thrown into a crowd in the city of Taiz, al-Jazeera news network reported.