A 48-year-old man and a 30-year-old man were detained by Scotland Yard's Counter-Terrorism Command under the UK's Terrorism Act this morning, after a search at an address in Newport, South Wales, the Metropolitan Police said.
Yesterday evening, a 25-year-old had been arrested in Newport.
On Saturday an 18-year-old man was detained at Dover port, and a 21-year-old was arrested in Hounslow, west London.
Both are still being questioned in custody, police said.
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A homemade "bucket bomb" partially exploded in a train at Parsons Green station in west London, injuring 30 people during rush hour on September 15.
The latest arrests are also under section 41 of the UK's Terrorism Act, as in the case of the all suspects in the case.
The arrests were carried out by the Met's Counter- Terrorism Command, supported by Gwent Police and the Welsh Extremism and Counter-Terrorism Unit.
"This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday," Metropolitan Police Commander Dean Haydon said.