Police said Wednesday they had arrested a seventh suspect after the discovery of the deadly toxin ricin in a north London apartment.

London's Metropolitan Police said they had arrested the 33-year-old man, whom they did not identify, on Tuesday. He was being held at a police station in central London, they added.

They gave no further details on the arrest.

Investigators apprehended six men Sunday in north and east London and said materials seized at a flat in the Wood Green neighborhood had tested positive for trace amounts of ricin, a virulent poison.

Doctors throughout Britain have been warned to watch for signs that Britons have been poisoned by the toxin, which is twice as potent as cobra venom. There is no antidote, and victims die within days.

None of the men, who were arrested under the Terrorism Act, has been charged. Scotland Yard says they are of North African origin but declined to specify which country or countries they come from and refused to confirm media reports they are Algerian.

Police gave no details on where the seventh suspect was from.

Ricin is derived from the castor bean plant, which is grown around the world, and is relatively easy to produce. It has been linked in the past to Usama bin Laden's Al Qaeda terror network and Iraq.

Prime Minister Tony Blair said Tuesday that the find highlighted the threat posed by weapons of mass destruction.

"As the arrests...show, this danger is present and real, and with us now, and its potential is huge," he said.

A government minister urged the public Wednesday to be "alert but not alarmed" about the discovery.

"It's understandable that people are concerned," Home Office minister Beverley Hughes told the British Broadcasting Corp. "But we need to keep it in perspective. People need to be alert, but not alarmed and panicked."

The Daily Mirror newspaper showed a skull and crossbones against a map of Britain on its Wednesday front page under the headline "It's here." The Sun, Britain's biggest circulation daily, said the discovery revealed a "factory of death."

"This makes us feel really scared. We are only three or four yards away from it," said Ismail Ucur, 19, who lives opposite the apartment where the poison was found.

"We should have been told what was going on. We saw the police and then people in protective gear and we asked them what was happening but they didn't tell us," Ucur said.

"It's terrifying to think it was on our doorstep," added Ali Goren, 20, who works at a cafe next to the apartment.

The public health director for London, Sue Atkinson, urged people not to be alarmed. "What has been found is a very small amount of this and it's quite difficult to perhaps use it for mass destruction," she told the BBC.

In Washington, U.S. officials said Tuesday that no al-Qaida links had been established in the London arrests, but the investigation for ties continued.

Police gave no information about the suspects, saying only that they were in their late teens, 20s and 30s.

They refused to confirm reports that investigators had discovered a makeshift laboratory in the Wood Green apartment, or that they were looking for more ricin elsewhere.

U.S. officials said in August that the Islamic extremist group Ansar al-Islam had tested ricin along with other chemical and biological agents in northern Iraq, territory controlled by Kurds, not Saddam Hussein. The group is allegedly linked to al-Qaida.

United Nations weapons inspectors who left Iraq in 1998 listed ricin among the poisons they believed Saddam produced. U.S. troops also found traces of the substance at suspected al-Qaida biological weapons sites in Afghanistan.