Italian magistrates have opened an investigation after a Gambian man drowned on 22 January in Venice's Grand Canal in front of onlookers who filmed the incident, laughing and shouting racist comments.

More than 181,000 migrants arrived on the Italian coast by boat in 2016, most of them from sub-Saharan Africa. Immigration authorities said this represented an 18% rise compared with 2015.

Onlookers watch and refused to jump into the icy waters to save Pateh Sabally, a 22-year-old refugee from the Gambia, as he appeared to be drowning in Venice's famous canal, Reuters news agency reported.

The Times newspaper, which shared a video of the incident apparently filmed on a mobile phone on Sunday, said tourists "laughed, shouted and made racist remarks" as Sabally, who had reportedly arrived in Italy two years ago after fleeing his home country, drowned.

"Go on, go back home," one of them is heard shouting in the amateur video.

Reports indicated that people threw at least three life rings into the water near Sabally, but he did not appear to reach for them. The footage clearly shows onlookers speculating that the refugee may have wanted to take his own life. "He is stupid. He wants to die," someone is heard saying in the video.

Local media reported that the young man, who had a permit to stay in Italy, had travelled to Switzerland to look for work to be closer to relatives in Germany, but was allegedly sent back to Italy.

The incident comes at a time when anti-immigration and even anti-refugee rhetoric is rising in Europe.

Migrants pay thousands of dollars to smugglers in Libya and other refugee transit hot spots for the privilege of packing on to dangerous and often unseaworthy boats to make the perilous journey to Europe.

The smuggling of migrants to Italy and onward to northern Europe is big business, with sub-Saharan Africans pay $400 to $700 per head for the voyage from Libya, according to the International Organisation for Migration (IOM).