National police chief Khalid Abu Bakar said the suspects were arrested in a western suburb of the capital Kuala Lumpur.
"The suspects are believed to be planning to cause civil unrest in the country," Khalid said in a posting on the official Malaysia police Facebook page.
He gave no details of the items seized or the backgrounds of those detained, nor did he make any link to the diplomatic gathering. Police counter-terror officials could not immediately be reached for comment.
Over the past year the Muslim-majority nation has announced a series of arrests that authorities say have involved suspected supporters of the extremist Islamic State (IS) group.
Malaysian police say dozens of its citizens have gone to Syria to join the brutal IS jihad, and have warned of radicals returning to stage attacks on home soil.
In early April police said they arrested 17 IS-inspired militants who were plotting to kidnap unspecified high-profile figures, rob banks and launch terror attacks.
The law has drawn fire from the parliamentary opposition and human rights groups, who say the long-ruling government has a history of using security laws against political opponents.
The government - which is steadily losing ground in parliament - has promised the law would not be abused for political ends.
But opposition lawmakers complain that authorities have shared no details with them on the dozens of claimed arrests or the extent of the purported terror threat.
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