The letters include incitement to rip off women's headscarves, and to verbally and physically attack Muslims.
A source at the scene said they were anxious the incident was linked to the sick "Punish a Muslim Day" letters being sent round, revealed by The Sun last week.
Two people were taken to hospital as a precaution after a package leaking a suspicious liquid was sent to a Labour MP's office.
Four Muslim Labour MPs have been sent suspicious packages over the past 24 hours and received copies of a "Punish a Muslim" letter that advocates violence against Muslims.
Huq, who said she was in a select committee meeting at the time of the incident, has been MP for Ealing Central and Acton since 2015.
MPs have condemned letters encouraging recipients to take part in "Punish a Muslim Day" as "abhorrent".
"I have been contacted by a number of people who are anxious about the racist literature that was sent to my office today", Yasin wrote on Facebook.
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"There is a definite pattern", she said, after two other Muslim lawmakers reported receiving similar packages on Monday.
The leaflets, reported in West Yorkshire, Midlands, London, Cardiff, Leicester, Sheffield and Birmingham, carried an image of a dagger with the letters MS, indicating links to so-called Muslim Slayer which targeted mosques in London and the United States past year with similar threats, the Mirror reported. "Among my colleagues, people have shown support but they are outraged".
A Metropolitan police spokesman said: 'Police were called at 9.49am on Tuesday, March 13 to reports of a suspicious package at Norman Shaw Buildings, Victoria Embankment, Westminster. "One person attended hospital as a precaution". "The police are investigating the content of the letters to identify any potential link".
Counter Terrorism Policing North East said it had received reports of "potentially malicious communications sent to individuals across the UK".
City A.M. understands all staff at Parliament have been emailed warning them to be on alert.
The letters sparked shock and fear in communities around the U.K. Naz Shah, a parliamentarian for Bradford West-one of the communities that received the letters-called on her constituents to report any suspicious behavior to the police immediately.