Police suspect a far-right motive after two improvised explosive devices detonated outside a mosque and congress centre in Dresden just before and after 10pm on Monday.

No one was injured during the two attacks in the eastern German city, but the mosque’s 46-year-old imam, his wife and their two children were inside the building when the first bomb went off.

Imam Hamza Turan told Sächsische Zeitung newspaper that six bottles filled with explosive gas had been found at the site of the attack. “They attacked us because they hate us, because we are Muslims”, Turan’s 10-year-old son told the local paper.

The second improvised device exploded on a terrace facing the river Elbe, between the International Congress Centre and the Maritim hotel. The hotel bar was evacuated as a result.

Horst Kretzschmar, the president of Dresden’s police force, said that even though no group had claimed responsibility it was assuming a “xenophobic motive” behind the attack.

Kretzschmar said investigators were also assuming a link to the city’s planned festivities around the Day of German Unity next week. Between 1-3 October, Dresden is due to celebrate the 26th anniversary of German reunification with concerts and performances in the city’s historic town centre. The chancellor, Angela Merkel, and the president, Joachim Gauck, are among those expected to attend.

Gauck is due to host a reception on Monday for delegates from across Germany at the International Congress Centre.

Police say they have stepped up security at a number of venues around the city, including Dresden’s three mosques, a prayer room and a inter-faith meeting place.

The mayor of Dresden, Dirk Hilbert, said: “Such an act is not a protest or the expression of an opinion. Such an act is a crime”.

The attacks are a blow to authorities who had hoped the festivities would improve the city’s reputation after Dresden grabbed international headlines for its large-scale anti-refugee protests.

Dresden is the cradle of the anti-Islam Pegida protest movement, whose rallies attracted about 25,000 supporters in January 2015. In recent months, support for the movement has ebbed away, with a rally in September attracting up to 2,300 protesters.

Tourism has suffered in recent months, with visitor numbers in July down by 10% compared with last year.

Stanislaw Tillich, the Christian Democrat president of Saxony, said on Monday he hoped the festivities would help to present the region in the former GDR as “a modern, cosmopolitan and hospitable country”.