Most of the arrests were made in raids in London
Six were held in London and one in Greater Manchester.
Sources said all the detainees were Algerians and the majority were among the eight men cleared in April of involvement in a ricin poison plot.
The home secretary said he ordered the raids following "detailed submissions" from the police and security services.
New offences
The operation came as Charles Clarke outlined his long-awaited anti-terror proposals.
Measures include extending the time that police can hold terrorist suspects for questioning without charge - from two weeks to three months.
There are also proposals to make it illegal to prepare for terrorist acts, train for them or indirectly incite them.
Two new criminal offences of glorifying terrorism and disseminating terrorist material would also be created.
The home secretary is seeking cross-party consensus in an effort to get new laws through Parliament quickly in the wake of the London bombings in July.
But opposition parties have already expressed reservations with parts of the plans.
'Potential attacks'
Mr Clarke told BBC News: "As, unfortunately, we saw in July, there are individuals who are going about the course of trying to threaten the civil liberties of this country by terrorist or potential terrorist attacks.
"To the extent we know about those... it's our job to defend ourselves against that, as vigorously as we possibly can."
He refused to comment on the individual cases of those detained on Thursday.
The Home Office said the seven detained were being held in "secure prison service accommodation" under the 1971 Immigration Act. They have the right to appeal.
The same powers were used in August to detain another 10 foreign nationals said to pose a threat to national security, including radical Jordanian cleric Abu Qatada.
We already have pretty broad powers to deal with people thought non-conducive to the public good or a threat to security
Shami Chakrabati
Liberty director
Clarke backs detention calls Do we need new terror laws?
Of the eight men cleared of involvement in the alleged ricin plot in April, four were tried and found not guilty and a further four were acquitted after the prosecution offered no evidence against them.
Another man, Kamel Bourgass, 31, was found guilty at the Old Bailey of plotting to spread the substance and other poisons on UK streets, and jailed for 17 years.
A spokesman for civil rights group Liberty said: "At this stage it's not clear why these men have been detained today, but we are very concerned that as the majority appear to be Algerian, they are likely to be deported there.
"International observers and the United Nations have repeatedly pointed out violations of human rights by Algeria.
"We think it is likely these men will appeal against their deportation on that basis."
The group said that if those detained are men previously acquitted of involvement in the ricin plot, it would send out the "wrong message" about the justice system.
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In dawn raids ordered by the UK Home Secretary, Charles Clarke, seven people have been detained in London and Greater Manchester.

The seven, who are believed to all be Algerian, are thought to include some of those allegedly involved in an earlier ricin poisoning plot.

The Home Secretary ordered the raids based on information provided by police and other security forces.  Those seized may appeal against deportation, but are deemed a "threat to national security" by the Home Office.  According to officials, those detained were done so using powers granted under the Immigration Act of 1971, not under the recently passed legislation aimed at removing radical muslim clerics from the country.