Russian rescuers have found the body of a 18-month toddler who was swept away in the sewage system with his mother when the pavement buckled under them in the central city of Bryansk.

The emergency service said the mother was walking her child in a stroller through the city's main square on Sunday when the pavement suddenly gave way because of a ruptured hot water pipe.

The baby -- named by the Russian media as Kirill -- fell out of his stroller and vanished while the woman's policeman husband managed to pull her out of the gaping four-square-metre (45-square-foot) hole using a rope.

Police had initially found torn pieces of the toddler's clothing before recovering his body some 28 hours later nearly six kilometres (3.5 miles) away from the original spot. The 26-year-old mother was taken to hospital in shock.

Several mourners dropped off stuffed toys at the site of the tragedy while a picture of the baby with a black ribbon appeared near the hole.

State television said the smooth pavement on the square had recently been replaced with paving stones that some Russian cities are starting to use to give the streets a more European feel.

Investigators opened a negligent homicide inquiry into the incident while the local governor vowed to commit more local money to infrastructure repairs.

"We have to start rebuilding things because otherwise, God forbid, this might happen again," regional head Nikolai Denin said in televised comments.

Underground pipes carrying hot water occasionally burst in Russia because of extreme temperature swings that cause cracks in the system and release scalding steam that buckles the pavement above.