The detentions reflect a deepening isolation of Hamas, even by Arab countries, since it formed a Palestinian government last March.
The arrests are connected to the discovery on April 18 of a cache of weapons, including Iranian-made Katyusha rockets and LAW anti-tank missiles that were allegedly smuggled into Jordan and stockpiled there, government spokesman Nasser Judeh said.
Since the discovery, 20 Hamas militants have been detained, and investigations could reveal more people involved in a plot "targeting locations, military and civilian officials," Mr Judeh said.
The government suspects that more hidden weapons have yet to be found, constituting a "great danger to Jordan's national security," he said.
Hamas denies charges
In an implicit reference to the Hamas-led government, Mr Judeh warned that Jordan will "hold whoever responsible for - God forbid - any action or incident" in the kingdom. Hamas officials have denied the accusations.
The alleged plot has further hurt already long-strained ties between Jordan and the radical Palestinian movement. A day after the cache's discovery, Jordan cancelled a planned visit to Amman by Palestinian Foreign Minister Mahmoud Zahar, a top Hamas leader.
Hamas had hoped for aid from Arab and Islamic nations to fill the gap, but Arab governments have largely failed to fulfill promises of financial support.
The aid that has been offered, most of it from Iran and the Gulf state of Qatar, is stuck in an account in Egypt because banks refuse to transfer it, fearing retaliation from the United States.
Amman's relations with Hamas have been strained for years. Jordan, a longtime US Arab ally and a peace partner with Israel, once expelled the current Hamas boss, Khaled Mashaal, for his activities.
Jordan has called on Hamas to accept an Arab peace plan, which entails full recognition of Israel in exchange for full withdrawal from territory captured in the 1967 and 1973 Arab-Israeli wars.
Palestinians briefed
The weapons plot has raised speculation by Hamas supporters here that Jordan was bowing to US pressure to further undermine Hamas.
The Islamic Action Front, Jordan's largest opposition group and a known Hamas sympathiser, accused the government of exaggerating the arms discovery.
Mr Judeh said that state television would air confessions by the detained activists to address public suspicions about the accusations.
Jordanian authorities have briefed five top Palestinian security officials of the evidence against the Hamas militants. The team was sent by Palestinian leader Mahmoud Abbas, of Fatah, not the Hamas government.
Mr Judeh said the Palestinian team was being shown "secret documents, information and evidence which implicates (Hamas) beyond doubt".
He said Hamas had been seeking to "recruit elements to operate" in the kingdom and to bring activists from the Palestinian territories into Jordan "to send them to Syria and Iran for training on intelligence, security and military activities".