A jackhammer pounded large steel beams side by side into the sandy soil on the Egyptian side of Gaza's border, putting in place an underground wall that could shift the balance of power in this volatile area.

Once completed, the steel barrier would cut off blockaded Gaza's last lifeline and _ by slicing through hundreds of smuggling tunnels under the nine-mile (14-kilometer) Gaza-Egypt border _ could increase pressure on the territory's Hamas rulers to moderate.

The Islamic militants have so far shown little willingness to compromise in power-sharing talks with their Western-backed rivals or in negotiations on a prisoner swap with Israel. Their hold on Gaza is at least partly dependent on supplies and cash coming through the tunnels.

On Monday, workers operated huge machines just behind the Egyptian border line, offering a rare glimpse at what the wall is made of.

A drill pierced holes in the soil, a crane lifted steel beams into position and a jackhammer drove them into the ground as several workers could be seen welding.

Egyptian troops in four armored personnel carriers with mounted machine guns guarded the crew. In the past, shots were fired several times from Gaza at the workers, though no one has been hurt.

Hamas guards watched from a nearby position, some shouting insults at an Egyptian soldier who poked his head out of his armored vehicle.

Hamas leaders are furious about the border wall and are seeking to rally Arab and Muslim public opinion against Egypt. On Sunday, demonstrators marched outside Egyptian embassies in Jordan and Lebanon, holding posters showing Egypt's president with Israel's Star of David on his forehead.

Hamas has also marshaled Muslim scholars who decreed that the barrier is "haram," or religiously forbidden. The scholars were responding to a statement by Al-Azhar University, Egypt's prestigious Islamic seat of theology, which reached the opposite conclusion last week.

Gaza's borders have been virtually sealed since June 2006 when Hamas-allied militants captured an Israeli soldier, Gilad Schalit. The blockade by Israel and Egypt intensified a year later when Hamas overran Gaza, seizing the territory from Palestinian President Mahmoud Abbas.

The blockade has evoked intense international criticism, but Israel justifies it by claiming that supplies to Gaza could end up in the hands of violent militants.

In response to the stifling closure, Gazans dramatically expanded smuggling from Egypt to bring in commercial goods, along with weapons and cash for Hamas.