(Updates with capture of compound)
LONDON Aug 23 (Reuters) - The fortified Tripoli compound stormed by Libyan rebels on Tuesday was the seat of Muammar Gaddafi's political power and the principal base of loyalist fighters trying to rescue his 42-year-old rule.
Gaddafi loyalist fighters confronted with an invasion of rebels since the weekend had sought to use his Bab al-Azizyah bastion as a springboard from which to carve out a loyalist zone and chip away at rebel control of adjoining neighbourhoods.
The encampment, surrounded by metre-thick olive green walls, is believed to sit atop a network of tunnels and bunkers that lead to adjoining districts, including possibly a subterranean route to the coast.
"This is Gaddafi's Pentagon," said Noman Benotman, a senior analyst at the British think tank Quilliam and a former Libyan Islamist opposition guerrilla commander.
He said tunnels forking out from the complex to nearby districts gave his fighters precious access to supplies. Many could have fled before the final attack.
Long the seat of Gaddafi's power as well as his main Tripoli home, the barracks may well be the final battleground of Libya's war. Even if Gaddafi is not there, its loss inflicted a potentially crippling symbolic defeat.
If Gaddafi is at the six sq-km (2.3 sq mi) base, located south of Tripoli at the northern end of the Airport Highway, there is a fair chance he is in a tunnel under it rather than in a fortified room above ground.
The compound has often been targeted by NATO air strikes, but it is still defended by tanks and snipers, a fact that suggests Gaddafi or at least some key aides are nearby.
The site was dotted with tents, residential buildings, security encampments, and the cratered remains of a house bombed by U.S. warplanes in 1986 and kept in ruins as a memorial.
Rebels fired their guns in celebration before the building.
Umar al-Hariri, a military official of the rebel Libyan National Transitional Council, told Al-Sharq al-Awsat newspaper in June that the barracks were linked to underground tunnels up to 30 km (20 miles) long, some of which led to the sea.
Built in the era of King Idris, overthrown by Gaddafi in the 1969 putsch that brought him to power, the site was reinforced in the 1980s using an array of foreign contractors.
(Reporting by William Maclean)