RAFAH, Gaza Strip -- 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 Gaza-Egypt border -- could increase pressure on the territory's Hamas rulers to moderate.
Since Israel's withdrawal from Gaza in 2005, Hamas is believed to have stepped up its weapons imports considerably, and Israel has struck hard to stop the flow of arms. But the underground passages also pose a threat to Egypt, which increasingly is concerned about an Islamic militant regime on its doorstep that could spill into its territory and incite violence.
The Islamic militants so far have 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 also has 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.
Today, nearly 400 tunnels run under Gaza's border with Egypt, employing 15,000 people and bringing in $1 million in goods a day, said Issa Nashar, the Hamas mayor of the Gaza border town of Rafah. The municipality supplies electricity and levies $2,500 in taxes per tunnel, he said.
Large white tents mark the tunnel entrances on the Gaza side.
During a tour Monday, rows of tents were visible along most of the border. A stretch of sandy soil, about 200 yards wide, runs between the tents and the first Egyptian demarcation, in some places a low stone wall and in others a line of rusty steel containers.
The tunnels run under the border and emerge about a half-mile away on the Egyptian side, the exits often disguised by homes.
Construction of the anti-tunnel wall is believed to have started sometime in November, though Egyptian officials initially would not discuss the project and still decline to provide details. In recent days, as opposition to the wall mounted, Egypt's leaders have struck a defiant tone.
"Egyptian borders are sacred, and no Egyptian allows any violations in one way or another," Egyptian Foreign Minister Ahmed Aboul Gheit said last week.
It's impossible to gauge how much of the wall already has been completed, but smugglers watch the construction with growing concern. Tunnel operators standing near Monday's work site said they have not been directly affected so far but fear the day when they have to stop working.
Profits from the tunnels are still considerable. A 36-year-old former taxi driver said he makes $100 a day, a large sum for Gaza, by pumping fuel from Egypt through his tunnel.
Amid the uncertainty, rumors are running wild. Many here believe Egypt plans to flood the area and already are scheming to make their tunnels waterproof. Mr. Nashar, the Rafah mayor, said enterprising smugglers have managed to cut pieces off the underground wall.
Others have raised the possibility that the smugglers simply might dig deeper, going below the underground wall.
Two years ago, Hamas militants cut down a metal border wall that had been erected by Israel, enabling tens of thousands of Gazans to pour into Egypt until the border was resealed.
During Israel's 38-year military control of Gaza, Israel tried in vain to halt the smuggling, including tearing down houses along the border and blowing up tunnels.
In Israel's three-week military offensive against Hamas last winter, warplanes repeatedly bombed the border area, causing some damage but failing to close down the tunnels.
The wall construction marks the highest profile attempt by Egypt to halt the smuggling and seems to have struck a nerve, judging by Hamas' angry protests.
Hamas officials portrayed Egypt as doing the bidding of Israel and the United States and even hinted at another border breach.
"I'm telling you, the people, they want to live and they want something to eat. They may do everything they can," Ehab Ghussein, a spokesman for Gaza's Interior Ministry, said Monday.
"But we don't hope to reach that point."
Salah Nasrawi in Cairo and Diaa Hadid in Jerusalem contributed to this report.