On the top of floor of a nondescript little building here, Mousa Abu Marzook's office is shut tight. A crust of red sealing wax fixes the door to its frame, and a scrawled sign is posted on the outside: "Closed by order of the General Prosecutor for State Security."

Marzook, a dapper businessman and an official with the militant Islamic Resistance Movement, known as Hamas, was visiting Iran in late August when Jordanian security agents raided his office here and sealed it. When he tried to return to Jordan last month in defiance of an arrest warrant, he was seized at the airport, barred from entering the country and placed on a flight to Yemen.

The raids on Marzook's offices and three others, and the arrests of 21 other Hamas activists in Jordan, were ordered by King Abdullah, the 37-year-old monarch who came to the throne in February on the death of his father, King Hussein. After just eight months in power, Abdullah faces his first major test in sustaining the crackdown against Hamas, a hard-line group whose underground military wing has carried out bloody terror attacks against Israel.

For the king, it is a personal as well as a political test. A career army officer educated in America and England, he had no inkling he was in line for the throne until his father shuffled the deck of succession only weeks before his death. An avid parachutist, frogman and pilot, he had only recently been promoted to the rank of major general, in command of the Jordanian army's special operations branch. His business was commando raids, not political intrigue.

Now, having barely consolidated his power, Abdullah has scrapped his father's tolerance of Hamas, declared war on an organization that enjoys considerable popular sympathy, particularly among Palestinians in Jordan, and seems determined to crush it. In doing so, he may also be subtly redefining Jordan's relations with Yasser Arafat's Palestinian Authority and refining the kingdom's supporting role in the Middle East peace process.

In an interview with a group of Western journalists today, the king made his position clear: "Hamas offices will be shut down in Jordan," he said.

The king heads to Washington later this week and is scheduled to meet with President Clinton, Secretary of State Madeleine K. Albright and others next week. He said the main item on his agenda will be advancing Israeli-Palestinian peace talks, following what he called the euphoria that greeted the election of a new government in Israel this year. "My greatest fear is what happens if nothing happens in the next months," he said.

The king's move against Hamas carries risks of a backlash, according to analysts. By all appearances, however, he timed the crackdown carefully. The raids and arrests were carried out just as Israel and Arafat were reviving their long-dormant efforts to reach a broad peace settlement; the last thing either needed was the threat of terrorist attacks.

Hamas has suffered setbacks recently. Israel killed two of its top leaders a year ago; Syria has warned it to drop armed activities against Israel. And Arafat's Palestinian Authority has arrested dozens of activists in the West Bank and Gaza Strip. Jordan, loath to be the odd man out, worried that it might become a haven for the group. Moreover, the government feared that Hamas was gradually radicalizing Jordan's own broadly popular, and largely peaceable, Islamic movement.

Still, the move came as a surprise to many Jordanians. Hamas has a large following among Palestinians in the West Bank and Gaza and constitutes the main opposition to Arafat. King Hussein, who often clashed with the Palestinian leader, saw the group as a useful stick, and he had allowed it to function in Jordan since 1991. In 1997, he rushed to the group's defense when Israeli agents tried unsuccessfully to assassinate one of Hamas's chief Jordanian officials on the streets of Amman.

Abdullah, by contrast, seems more interested in helping Arafat than competing with him. But while the crackdown on Hamas is a blessing to the Palestinian leader, it has prompted howls of protest in Jordan, especially from the country's influential and popular Islamicists.

"The government thinks final peace talks [between Israel and the Palestinians] required the silencing of every voice and every bullet directed against Israel," said Abdel Majeed Thneibat, chairman of Jordan's Muslim Brotherhood. "[But] if they're put on trial, the political opposition in Jordan will expand, and the supporters of peace will lose ground. There will be a backlash."

Whether there will be a trial is unclear. The government appears eager to avoid one and raided the leaders' offices while they were in Iran, evidently hoping they would not return.

But two of the group's leaders--who, unlike Marzook, hold Jordanian citizenship--returned and were jailed along with 19 other Hamas activists. Both are fairly well known. One, Khaled Meshal, was the target of the botched Israeli assassination attempt in 1997. The other, Ibrahim Ghosheh, a frail, well-educated engineer, was the group's main spokesman.

Marzook was deported from the United States to Jordan in 1997 after being detained in a federal prison for almost two years.

If they are tried on charges of endangering Jordan's security, the government could risk turning them into martyrs. One recent newspaper advertisement featured photographs of four of the group's leaders under the headline: "The conscience of a people and the nation."

The government cannot easily back down. "To a novice like King Abdullah who hasn't flexed his muscles yet, it would be a big mistake to free [the Hamas leaders]," said Radwan Abdullah, a Jordanian political scientist. "It could be seen as a sign of weakness."

The Hamas leaders' return seemed a direct challenge to the king and may have strengthened his resolve. In the view of many analysts, he cannot afford to lose his first real political test of wills.

Jordanians speculate that the government would prefer to release the Hamas leaders, perhaps on condition that they go into exile. If that is the government's demand, they may have little choice; in the state security courts, they could face a sentence of up to 15 years in prison.

CAPTION: King Abdullah, right, has moved against Hamas, which enjoys sympathy among Jordan's Palestinians.