SNP treasurer Colin Beattie has been arrested in connection with a police investigation into the party’s finances.
Police Scotland said the 71-year-old has been taken into custody and is being questioned by detectives.
The arrest of Mr Beattie, who also serves as an MSP for Midlothian North and Musselburgh, is the second in relation to the investigation into how more than £600,000 in donations to the party earmarked for an independence referendum had been used.
Police said the 71-year-old was arrested “in connection with the ongoing investigation into the funding and finances of the Scottish National Party.”
The force said a report will be submitted to the Crown Office.
Earlier this month, former party chief executive Peter Murrell was arrested and questioned for more than 11 hours before being released without charge “pending further investigation”.
Mr Murrell is the husband of former first minister Nicola Sturgeon.
A spokesperson for the SNP said: “We have no comment on a live police investigation.”
Mr Beattie was the SNP’s national treasurer between 2004 and 2020, losing the position to MP Douglas Chapman.
However, Mr Chapman quit the post in 2021, claiming he was not being given the required information to carry out the job.
There was no sign of police activity outside Mr Beattie’s address in Dalkeith, Midlothian, on Tuesday morning, or outside his constituency office in the town.
Labour leader Sir Keir Starmer said the SNP is in a “real mess”.
During a visit to York University, he was asked about the arrest of Mr Beattie.
“Obviously there is a criminal investigation going on, so there is only so much I can say.
“But after 16 years in power, it has descended to this – a real mess – and those being let down are Scottish voters, who are entitled to better than this.”
Meanwhile, Scottish Conservative chairman Craig Hoy MSP called for Mr Beattie, Mr Murrell and Ms Sturgeon to be suspended from the SNP.
He said: “This extremely serious matter is escalating by the day and everyone in the SNP has a duty to be as transparent as possible about what they knew and when.”
Scottish Labour deputy leader Jackie Baillie said: “This is yet another deeply concerning development and the Police Scotland investigation must be allowed to proceed without interference.”