About 40,000 Paraguayans have been forced to evacuate their homes in recent weeks due to rising waters.
Officials say that more than 11,000 of the evacuees live in the capital Asuncion.
Although they were moved to higher grounds at military bases, public squares and roadsides, some have complained of inefficient government response.
“The government treats us like animals,” said evacuee Cecilia Salgueiro.
“It should have rolled up its sleeves and built a defensive [barrier] along the coast” to avoid the waters from reaching low-lying flood-prone slums, Salgueiro said.
The emergency declaration covers the hard-hit Neembucu province, about 300km south of Asuncion.
Most of its territory contains wetlands. Governor Luis Benitez said in a tweet that “the situation is difficult and critical for six of our towns”.
Other evacuees are spread throughout the landlocked country along the rain-swollen Paraguay River, which originates in the Brazilian state of Mato Grosso do Sul.
Its normal level is 4 metres, but it has reached 6.75 metres in the capital due to unusually heavy rains since March.
“We’re not getting any help from the National Emergency Office,” complained Alejandro Ortiz, who is staying on the first floor at his home after the ground floor flooded.