Greece bans large gatherings for 'health' reasons
"All public gatherings of over 100 people (are restricted) until February 1," the police said in a statement.
Student groups have been demonstrating for weeks against plans to introduce a special police force to guard universities.
Another protest on the issue is to be held on Thursday.
A protest has also been called Friday in support of the jailed top assassin of far-left extremist group November 17, Dimitris Koufontinas, who went on hunger strike earlier this month to demand a prison transfer.
Anarchist groups are also scheduled to protest at the Athens city hall Monday against an urban redevelopment plan.
The police said large protests posed a "danger of further boosting the spread of the coronavirus."
In a follow-up statement, the police denied the ban was linked to the planned protests.
Greece remains under a national lockdown to curb the spread of the coronavirus, with movement restricted between regions and people required to give a valid reason for leaving their homes.
The ban was eased on January 18 to allow retail outlets to operate, and secondary schools are to reopen on Monday.
More than 5,600 deaths in Greece have been attributed to coronavirus since the start of the pandemic. Over 280 people are in intensive care.