Two boats landed across beaches in Kent and Sussex and 17 people were detained.
A “small number” of migrants are missing after two boats arrived on the Sussex coast.
Police said a total of 17 migrants have been detained following landings in rigid inflatable boats (RHIB) in Hastings and Winchelsea on Thursday morning.
But a number of them escaped officers and are being sought by Sussex police.
A force spokesman said: “Police believe that a small number of people may have left the beach and dispersed following the landing of two rigid inflatable boats containing suspected illegal immigrants in the Hastings area on Thursday morning.
“It is thought they probably crossed the English Channel from France overnight, coming ashore at Hastings, and further east near Winchelsea Beach.
“Seventeen migrants, including men, women and children, have been detained by immigration officials, but a few others from the Winchelsea landing are thought to have disappeared into the surrounding countryside.”
Detective Inspector Jon Gillings, of Hastings Police, said two of the migrants required medical treatment, and officers are concerned that others who left the beach may also need help.
There were also reports of a boat landing in Leas, Folkestone, in Kent, and nine migrants were found and detained.
A spokesman for the Home Office said: “Crossing the Channel in a small boat is a huge risk. The criminal gangs who perpetuate this are ruthless and do not care about loss of life.
“We are working closely at all levels with the French authorities to tackle this dangerous and illegal activity. In addition, Border Force cutters are patrolling the Channel and we have deployed equipment to detect migrant activity.
“In August, the home secretary and her French counterpart agreed to intensify joint action to tackle small boat crossings in the Channel. This includes drawing up an enhanced action plan to deploy more resources along the French coast to intercept and stop crossings.”
Members of the public reported a boat coming ashore at Winchelsea in Sussex, and witnesses said they saw 12 people, but only 11 were subsequently detained.
In Hastings, the border force were alerted to a boat with six people on board and all six men were detained.
An inflatable kayak with three men on board was intercepted in the Channel and all men were taken to Dover.
Police say anyone in the area who sees a stranger who is cold, wet, hungry, disorientated or needs medical aid, should report it to police by calling 999, quoting 276 of 03/10.