The waters of the Tiber in Rome reached a record level early on Saturday.
Mayor Gianni Alemanno was quoted by the ANSA news agency as saying the river reached its maximum level at 1am (0000 GMT). He said the situation had "stabilised", but was "not over yet".
A young man is feared drowned after falling into the river while taking a photo. Italian media reported he was Irish and had come to the Italian capital for a wedding.
Three boats were stuck under an historic bridge, the Ponte Sant'Angelo.
On Thursday the head of the civil protection force Guido Bertolaso said the level of the Tiber was at its highest for 40 years.
More rain is forecast for Sunday.