BENGHAZI, Libya—Rebels menaced Col. Moammar Gadhafi's stronghold from all sides Thursday, as insurgent commanders said they have sent troops for an offensive against Tripoli and residents of the capital prepared their first mass demonstration in days on Friday.

Just 30 miles west of Col. Gadhafi's shrinking base of power, antiregime forces battled for the oil-industry town of Al-Zawiya. On Thursday evening, opposition forces gained control of Misrata, a coastal city 130 miles east of Tripoli.

In Benghazi, the country's second-largest city and the hub of eastern Libya, a group of army colonels who recently defected said they are plotting the end their former commander's 42-year reign, which began to crumble last week as Libyans joined the antiauthoritarian protests roiling the Arab world.

"We have a plan to bring down Tripoli," Col. Tareq Saad Hussein, one of seven former colonels who have taken charge of rebel forces in Benghazi, said in an interview. "We will not stop until we liberate the whole country."

Speaking inside a downtown army administrative compound—now the rebel commanders' operations center—Col. Hussein said his team had already begun sending rebels west toward Tripoli in small groups to slip past forces loyal to Col. Gadhafi.