Ali Hattar was accused of "provocation" and detained after delivering a lecture critical of the US military and political involvement in the region, his lawyer said on Monday.
Security officials said Hattar, a member of the engineers union, was released after being held overnight.
But his lawyer Muhammad Abu Jabara said a prosecutor accused him of "provocation" and referred him to a lower court which deals with misdemeanours.
If found guilty, Hattar could face a six-month to two-year prison sentence and be ordered to pay a 20 dinar fine ($28), Abu Jabara said.
Earlier, a union statement said Hattar was detained and referred to the state prosecutor following an address he gave this month entitled Why We Boycott America.
Israel peace treaty
The lecture was organised by the Committee For the Protection of the Nation and the Struggle Against Normalisation, a Jordanian group opposed to normalising ties with Israel which has had a peace treaty with Jordan since 1994.
"Our lawyers have gone to see the prosecutor to understand the exact nature of the charges against Ali Hattar," the head of the engineers union, Wail Saqqa, said before Hattar's release.
"I only heard a realistic and historical presentation about what America is doing against oppressed people. It is occupying Iraq and backs the Zionists against our people in Palestine"
Jordan's Engineers Union head
"These charges are still not clear but it seems they are accusing him of slandering a foreign country," he said.
Saqqa said he heard the lecture and did not think it contained anything incriminating.
"I only heard a realistic and historical presentation about what America is doing against oppressed people. It is occupying Iraq and backs the Zionists against our people in Palestine," Saqqa said.
"They may consider this to be offending to a foreign country but we see it as a nationalistic stand."
Saqqa said Hattar's audience "could have concluded that they should boycott American goods".
Jordan's King Abd Allah is one of the US's key allies in the Middle East.
According to rights group Amnesty International, dozens of possible prisoners of conscience have been arrested in Jordan since the US-led invasion of Iraq last year.
Jordan has seen several anti-US
Most were arrested following demonstrations against the war.
Amnesty says there have been reports of torture and ill-treatment of detainees.
Jordan says it only arrested those who were guilty of provoking instability and it denies charges of ill-treatment.