WRAPUP 11-Rebels say Tripoli encircled; Gaddafi defiant

Aug 15 (Reuters) - * Rebels say they seize Garyan, encircle Tripoli

* Gaddafi urges people to 'liberate Libya'

* UN envoy in Tunisia where rebels, govt said to hold talks

* Gaddafi security official flies to Cairo

(Adds government spokesman on Garyan in paragraph 7)

By Michael Georgy

ZAWIYAH, Libya, Aug 15 (Reuters) - Libyan rebels said on Monday they had seized a second strategic town near Tripoli within 24 hours, completing the encirclement of the capital in the boldest advances of their six-month-old uprising against Muammar Gaddafi.

In a barely audible telephone call to state television overnight, a defiant and apparently isolated Gaddafi called on his followers to fight rebels he referred to as "rats".

Gaddafi's forces fired mortars and rockets at the coastal town of Zawiyah a day after rebels captured its centre in a thrust that severed the vital coastal highway from Tripoli to the Tunisian border, a potential turning point in the war.