Remnants of Munich's main synagogue, which was demolished by the Nazis in June 1938, have resurfaced -- much to the amazement of the city's Jewish community.

Construction workers in the southern German city made the discovery while working on the renovation of a weir on the Isar river.

It was on June 28, while carrying out maintenance work on the weir, a small-scale dam, that the team came across columns from the former synagogue, together with a stone tablet inscribed with the Ten Commandments.

Bernhard Purin, head of Munich's Jewish museum, told CNN that he was surprised to hear the news the following day.

"I never thought we would find anything from the old synagogue," he said. ''I felt happy and sad at the same time about this extraordinary find."

On the one hand, the stones represent "happy times" for Germany's Jewish community, he said, "when it was possible to build such a big and great synagogue." He added: "But it also represents a monument to the destruction of the Jewish life starting in 1933."

In November 1938, five months after the synagogue was destroyed, the Nazis unleashed Kristallnacht, a rampage of state-sponsored violence against Jewish businesses, synagogues and homes across Germany and Austria.

Some of the recovered masonry is "artistically decorated," according to Purin, who explained that the tablet with the Hebrew inscription of the Ten Commandments would have been positioned above the Jewish holy book, the Torah.

After the synagogue was leveled on Hitler's orders, the demolition firm, Leonhard Moll, stored the rubble at a site in the west of the city. A department store now stands on the plot once occupied by the synagogue.

It has now come to light that Leonhard Moll used the rubble for work to the weir in 1956. About 150 tonnes of the debris, from the synagogue as well as other buildings wrecked in the war, were dumped in the river as part of the project, Purin told CNN.

The area initially used as a dumping ground was bought by the city in the 1970s and some of the remaining rubble was used to create small hills.

Purin said there is "a good chance" other remnants of the synagogue could be there, but he added that "there are trees growing on the hills now and it won't be possible to dig the stones out."

According to Purin, the recent find included more than three-quarters of the tablet, which is in "quite good condition" despite having been underwater for more than 70 years.

"A lot of photos exist from the destroyed synagogue from the inside and outside so we are optimistic that we can say that the stone tablet originally came from the Ark (containing the Torah) on the eastern wall of the synagogue," he said.

What happens next is unclear.

"What we found is only a small part of the synagogue, it is only a fragment," said Purin. "It will become a long project to figure out which parts belong to which part of the former synagogue."

In an email to CNN, Charlotte Knobloch, president of the Jewish community of Munich and Upper Bavaria, described the former synagogue as "a majestic building," adding: "I was lucky to see it as a young girl before Hitler ordered it torn down in June of 1938. Given how ruthless and quick the demolition took place, I could not imagine any piece of it would ever be seen again -- until last week.

"Seeing parts of a synagogue that was thought lost for generations reappear like this is hard to comprehend. At the same time, we were devastated to learn that the building's remains were apparently used as filler material at a building site long after the Nazi era ended -- and by the same company hired to tear down the synagogue in the first place.

"Today, we are working in close cooperation with the city of Munich and hoping to have the pieces return to our community sooner rather than later."

Katrin Habenschaden, Munich's deputy mayor, said in a statement to CNN that it is the city's duty to return the artefacts to the Jewish community.

"'The extermination of Jewish citizens during the Nazi era began with the destruction of Jewish culture," she said. "The demolition of the main synagogue on Hitler's orders marked the beginning of exclusion, persecution and destruction. The fact that today we can find remains of the once cityscape-defining magnificent building is a stroke of luck and touches me deeply. Jewish life was and is an integral part of our city's history, present and future."