"If the outcome is a "yes" vote and a unilateral declaration of independence follows, the destabilising effect on the economies of Catalonia and Spain could be very large". The Catalan parliament then approved the plan on September 6. "Vote for the future of Catalonia!".
Spanish police have confiscated millions of ballots in recent days as part of a crackdown to stop the October 1 vote, which has been suspended by Spain's Constitutional Court.
Eighteen MSPs expressed "grave concern" at the actions of the Spanish state in a letter to Spain's prime minister.
And police have seized close to ten million ballot papers, as well as other items destined for the vote.
On Saturday, Prime Minister Rajoy declared that the independence vote "would mean liquidating the law" and vowed that the referendum would not go ahead.
The Madrid government has in recent weeks taken political and legal measures to prevent the referendum by exerting more control over the use of public funds in Catalonia and arresting regional officials.
The Catalan government says it will unilaterally declare independence within 48 hours of a "yes" vote.
The Catalan regional government has accused central authorities of imposing an "undeclared state of emergency".
The independence movement is widespread enough that it is unlikely to dissipate if the regional government fails to convert a "yes" vote into a split from Spain. A harsh crackdown by Madrid on Sunday would likely trigger protests that could fuel further separatist sentiment.
The euro could face hefty sell-offs in the coming weeks, as long as the possibility of Catalan independence looms.
The president's remarks mark a departure from the official position of the United States, which, as recently as Monday, was that a planned nonbinding Catalonia referendum Sunday to separate from Spain was an internal matter.
Political analysts and most politicians believe the standoff could be resolved by a renewed dialogue between Catalan and Spanish authorities that would lead to a better tax deal and increased infrastructure spending for the region.
Trump also offered prayers and support to those affected by deadly attacks last month that killed 16 people in Barcelona and a nearby town.