EUROPEAN intelligence services have warned that terrorists are plotting co-ordinated mass attacks across the continent this year, with scores of jihadists having returned from Syria already armed and in full training for strikes.

The warning came as one-year commemorations in Paris for the Charlie Hebdo terrorist attacks culminated in a tree planting ceremony at Place de la Republique, that became the rallying point for public outrage and outpouring of grief. French President Francois Hollande has presided over all of the commemorations during the week.

Several thousand people gathered in the square as Mr Hollande unveiled a plaque next to the oak tree in memory of all those killed by terrorism in France in 2015.

Veteran rocker Johnny Hallyday performed a short song and the army’s choir gave a rousing rendition of the Marseillaise. All those in the square had to go through physical security checks.

In the spirit of co-operation, Mr Hollande then made an unannounced visit toParis’ main mosque.

“The president had a short conversation and a moment of friendship and fraternity over a cup of tea,” a French presidency official said.

Mosques across France opened their doors to the public this weekend in a bid by the Muslim community to build bridges.

Australians though had a small role in yesterday’s ceremony with Sydney reverend Bill Crews contributing to the tree planting with a small pot of compost made from the thousands of flowers placed at the site of the Lindt Café terror attack in Sydney in December 2014.

He said the nations were tied both in grief and opposition to terror.

France has been in a jittery state for much of last year with several Islamic State-inspired terrorist attacks culminating in the death of 130 people in multiple shootings and bombings of cafes and a theatre last month.

Some of those responsible remain on the run and yesterday a counter terrorism official told French media 2015 “was nothing” compared to what was coming particularly for France and London this year.

“We are moving towards a European 9/11: simultaneous attacks on the same day in several countries, several places,” the unnamed official told French media agencies.

“A very co-ordinated thing. We know the terrorists are working on this.”

The official said ISIS was specifically training European units in Syria to send them back to Europe, battle hardened, for homegrown assaults.

“They have the necessary false documents, the mastery of the language, the sites, the weapons,” he said.

“We stop a lot of them, but it must be recognised that we are overwhelmed. Some will get through — some already have.”

Former analyst for France’s DGSE intelligence service Yves Trotignon said multiple European national threats were not new with Al-Qaeda having plotted similar attacks in 2010 but being thwarted before their cells arrived from Afghanistan and Pakistan.

“This type of multiple attack is part of the worst-case scenarios for 2016,” he said.

“I know that in European capitals, particularly London, specialised services are working on this theory.”

Brussels too has been jittery after it uncovered suicide notes written by members of a bikie group that had pledged allegiance to Sharia4Belgium, a banned group of hard line Islamists many members of whom are currently in jail. The plot allegedly involved killing policemen and soldiers who have been deployed across the Belgian capital for added security. Two men have been charged.