A man has been arrested by officers investigating a series of letters calling for a “Punish a Muslim day”, police have said.

The 35-year-old from Lincoln was being held in a West Yorkshire police station on suspicion of conspiring and soliciting to commit murder, as well as sending a hoax noxious substance and threatening messages, after being arrested by officers from the north-east counter-terrorism policing unit.

Police said on Tuesday that they had carried out searches at a residential property in south Lincoln and an office building in the city centre.

Officers investigating the letters previously said they had been circulated among communities in South Yorkshire, Leicestershire and London, as well as in West Yorkshire. Recipients allegedly included the home secretary, Sajid Javid, who was serving as communities secretary at the time, and a host of Labour MPs.

“We take religious hatred extremely seriously and my officers will be carrying out a full and thorough investigation in response to these reports,” Det Ch Supt Martin Snowden, the head of the north-east counter-terrorism policing unit, said as he appealed for information in March.

“These letters seek to cause fear and offence among our Muslim communities. They also seek to divide us. Yet, in spite of this, our communities have shown strength in their response to such hatred and in their support for each other.”

Police said after the man’s arrest: “We understand that people may be concerned following this activity. However, we would like to reassure communities that this activity is a result of an ongoing, intelligence led investigation and was not in response to an immediate threat.”