Officials announced plans to provide free camping facilities in Thessaloniki, as well as free transportation and access to city venues to host gatherings.
Greece, desperate to avoid a repeat of violence witnessed during meetings earlier in its presidency, insisted it wanted the demonstrators' opinions to be heard.
Some groups have been planning their rallies for two years.
"We will do whatever we can to allow them not only to demonstrate but also to convey their views. We want to hear society's voice," Foreign Minister George Papandreou said.
"These are ... useful opinions on the big issues of globalisation, fighting poverty, protecting the environment and genetically modified produce."
The leaders' summit, scheduled for June 19-20 in Halkidiki, a peninsula next to Thessaloniki, will grapple with the EU's draft constitution as well as other problems related to the major expansion next year, when Malta, Cyprus and eight eastern European countries will join.
Thousands of police officers are due to be deployed around the summit area, 77 miles south-east of Thessaloniki - on a narrow strip of land where road access is easy blocked.