The Warsaw mermaid, "Syrenka Warszawska" is cognate with siren, but she is more properly a fresh-water mermaid, half-woman and half-fish, called "Melusina". It is the symbol of city and is featured in Warsaw's coat of arms since the mid of 14th century.
The syrenka has traditionally held a silver sword although this does not appear on more recent versions. Today the Coat of Arms of Warsaw consists of a syrenka ("little mermaid") in a red field.
http://en.wikipedia.org/wiki/Coat_of_arms_of_Warsaw

Syrenka appears on a shield on every municipal building, on buses, trams and taxis. It can be also found in many locations in Warsaw, on buildings, walls and roofs, in the streets and parks, in signboards and gates.

Legend of the Warsaw Mermaid (by Artur Oppman):
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Once upon a time there were two mermaid sisters, who swam to the shore of the Baltic Sea from their home deep in the sea. They were very beautiful. One of them decided to swim towards the Danish straits, to the port of Copenhagen. The second swam up the Vistula River and came out of the water onto the sandy bank to rest. It was at the foot of today's Old Town and she decided to stay.
The fishermen living in this area noticed, that during their fishing someone was churning the waves of the Vistula River, tangling the nets and releasing the fish. But the mermaid enchanted them with her beautiful singing, and they did not harm her.
One day a wealthy merchant saw the mermaid and heard her lovely song. He quickly calculated how much money he could make displaying her at market fairs, so he captured her and imprisoned in a wooden shed without any water. A young farmhand heard the mermaid's cries, and with the help of friends he freed her at night. Grateful that the townspeople had defended her, the mermaid promised "when difficult times come, I will return and arm the city with a shield and sword." This is why the Warsaw Mermaid bears a sword and a shield to defend the city.