"This alert level indicates a serious threat of terrorist action and requires the extreme mobilisation of the security organs," the interior ministry said.
It cited "reliable intelligence information" but gave no details about any specific attack threat.
Morocco has been on alert since suicide bombings in March and April this year when five people killed themselves and a police officer.
|"It is the first time Moroccan authorities have used clear language and precise words about the terrorist threat" |
Four years ago, 45 people died in a suicide attack in Casablanca.
"It is the first time Moroccan authorities have used clear language and precise words about the terrorist threat," Miloud Belkadi, a political analyst, said.
Belkadi said he believed that Western security officials, including Robert Mueller, the head of the FBI, might have passed details to Morocco about an imminent attack during their recent trips.
"These visits and the language of al-Qaeda's Zawahri were indicating something serious may happen soon here," he said.
A video released earlier this week showed Ayman al-Zawahri, al-Qaeda's deputy leader, urging the overthrow of what he called "corrupt" Muslim governments.
The interior ministry urged Moroccans to be more vigilant and support the country's efforts to prevent attacks, adding that more police were being deployed to step up surveillance.
It warned that the security threat against the country would not be short-lived.
"In view of the long-term nature of the threat and the requirements of the struggle against terrorism, the operational capabilities of security forces have been reinforced as part of a three-year plan to upgrade human resources and equipment of the security services," the ministry added.