More than 2,000 migrants have set up camp in Paris at two sites - about 800, mainly Afghans, along the Canal Saint-Martin in the north-east of the city and about 1,500, mostly Eritreans and Sudanese, under an overpass on the Canal Saint-Denis, further north.
And warmer weather means that more may arrive, notably from Calais, where a heavy police presence has been deployed to prevent the return of the "Jungle" camp of migrants hoping to cross the Channel to Britain.
"If nothing is done, in two weeks there will be 3,000," Hidalgo told the AFP news agency.
She has had no response to her requests to central government to find shelter, she said.
Camps cleared last year
The greater Paris region has accommodation for 750 migrants, many of whom have applied for refugee status but will be deported if their applications fail.
Last August police removed 2,500 people from camps in an area near to the present camps.
On Wednesday UN human rights experts deplored an "inhumane situation" in Calais and elsewhere in northern France, urging the government to provide water and emergency shelters.
"We are concerned about increasingly regressive migration policies and the inhumane and substandard conditions suffered by migrants," UN special rapporteur on migrant rights Felipe Gonzalez Morales said in a statement from Geneva.
Controversial immigration bill
President Emmanuel Macron's government is proposing an immigration bill that criminalises unauthorised border crossings and draws a sharp distinction between "economic migrants", who will be deported more quickly, and those the government accepts as refugees, who will have faster access to asylum.
Some NGOs and activists believe it will mean thousands of deportations and the plan has even been criticised by MPs from the ruling parties.
The government has already dropped a proposal to deport failed asylum-seekers to third countries deemed safe.
"We're lying to the French by saying this is a temporary crisis that we'll fix with an immigration law," Hidalgo said. "Let's be practical: Find shelter for the people sleeping in the camps and then we can look at their situations."
France received a record 100,000 asylum applications last year and offered refugee status to around 30,000 people, official figures show. Forced expulsions numbered 14,900.