Libyan rebels have pushed into Tripoli's central Green Square after moving past the city's outer defenses and claiming to have captured two of leader Moammar Gadhafi's sons, including his one-time heir apparent, Seif al-Islam.

The International Criminal Court Sunday confirmed that Seif al-Islam is in detention. Opposition leaders also said Mr. Gadhafi's eldest son, Mohammed, had surrendered to rebel forces.

Rebel troops approaching from the west met little resistance as they raced through government positions outside the capital.

Earlier, they captured a military base just 27 kilometers from Tripoli run by the government's elite Khamis Brigade, commanded by another of Mr. Gadhafi's sons. Rebel fighters hauled away truckloads of weapons and ammunition from the captured base. Opposition forces also freed several hundred prisoners from a government jail as they marched to the capital.

Inside Tripoli, thousands of civilians celebrated, cheering long convoys of pickup trucks packed with rebel fighters shooting into the air. A rebel spokesman said insurgents sent some fighters into the capital by sea from the port of Misrata.

Witnesses in Tripoli reported widespread clashes for a second day between rebel “sleeper cells” and Gadhafi loyalists. Mass anti-Gadhafi demonstrations were also reported in four districts sympathetic to the opposition.

Libyan state television Sunday broadcast a series of defiant audio messages from Mr. Gadhafi. In the latest one, he acknowledged that opposition forces were moving into Tripoli and warned the city would be turned into another Baghdad. The Libyan leader said he would stay in the capital “until the end” to defend the city and called on supporters to help liberate it.

In another audio late Saturday, Mr. Gadhafi urged his supporters to “march by the millions” and squash the uprising. He dismissed the rebellion as an ill-fated attempt by “traitors” and “rats.”

A government spokesman said late Sunday that 1,300 people have been killed in Tripoli since midday. The claim cannot be independently confirmed.

In a political victory for the rebels, the Tunisian news agency TAP announced Sunday that Tunisia's government has officially recognized the Transitional National Council as the legitimate representative of Libyans.

U.S. President Barack Obama was briefed on the Libya situation by a senior security adviser. A U.S. spokesman said the White House believes “Gadhafi's days are numbered.”

Mr. Gadhafi has seen the areas under his control shrink significantly in recent weeks as rebels advance on Tripoli from the west, east and south after six months of fighting to end his four-decade autocratic rule.

NATO warplanes have been supporting the rebels by bombing pro-Gadhafi forces under a U.N. Security Council resolution authorizing military action to protect Libyan civilians from government attacks.

Meanwhile, two more Gadhafi aides appear to have defected. Officials with Libya's opposition National Transitional Council say the Libyan leader's former number-two, Abdel-Salam Jalloud, is in Italy Sunday after joining their ranks a day earlier.

Tunisian officials also said Libyan Oil Minister Omran Abukraa has decided not to return to Libya after a recent mission to Italy.