By Tim Gaynor

SIRTE, Libya (Reuters) - Onto a battlefield littered with bizarre homemade weaponry, Libya's ruling militia fighters have rolled out their weirdest contraption yet: a concrete and steel behemoth that's a cross between a bulldozer and a battleship.

The towering monster, which appeared Wednesday in Sirte to help capture Muammar Gaddafi's home town, has a battleship's pointed prow and portholes along its sides with steel covers that can be pulled down.

Clad in concrete sandwiched between steel plates, it is painted in the colors of the new national flag -- red, green and black. Writing on the bow declares "there is no God, but Allah, and Mohammad is his prophet."

Built onto a tracked bulldozer in then-rebel workshops in the city of Misrata, the new weapon is designed to smash through roadblocks and barricades.

It was meant to make its debut in the fight for Tripoli, but when the capital fell swiftly to the rebels, the souped-up bulldozer was never brought in to do battle. Until now.

"It's to clear dumped cars and shipping containers," said Gebril Ali, the burly driver from Misrata, a heavy plant operator in civilian life.

He expects to be shot at with bullets and rocket-propelled grenades, but reckons the vehicle can withstand that. Heavier weapons might be a problem though.

"Maybe I'm not coming back," Ali said, suddenly reflective. He then erupted into laughter that revealed several broken teeth. "But my mind's made up. I'm stubborn."

The vehicle is also manned by four gunners who have five heavy machine guns to fire and a tank gun mounted on top. Several AK47 assault rifles and rocket-propelled grenade launchers sit inside next to water bottles and bananas.

There is also a helmet full of hand grenades, just in case.

"We will be watching from the portholes and if we see them trying to flank us, we'll shoot them," said crew member Ali Abdullah, a 20-year-old student. Asked if he thought the contraption was safe, he said: "We have faith in God."

The driver's task is made all the harder by the fact that he cannot see out of the vehicle. A video camera mounted on the front was damaged by gunfire and he has to rely on two comrades peering out of slits at the front to guide him.

A commander wearing a gold-braided ship's captain's hat sat on top and shouted directions as Ali maneuvered up to the battlefront. The craft smashed into a lamp-post.

It then came to a halt next to a mosque, and all scrambled out to go pray.

Just beyond the mosque and a turn to the right is a street constantly strafed with small arms and rocket fire by Gaddafi's forces.

The men planned to drive the beast down the street and smash their way into the center of Sirte. A Reuters news team was invited to come along for the ride, but declined the offer.

Commanding a posse of pick-up trucks mounted with heavy machine guns, the man in the sea captain's hat, postman Lutfi al-Amin, plans to follow.

"When it clears the way, we will follow," Amin said. Asked about his headgear, he said: "It's my lucky hat!"

(Writing by Jon Hemming)