Former Borussia Dortmund defender Marc Bartra has condemned a banner held by Schalke supporters during Saturday's Revierderby that apparently showed support for the bomb attack on Dortmund's team bus two years ago. 

Schalke won 4-2 at Signal Iduna Park on Saturday, as Dortmund had captain Marco Reus and defender Marius Wolf sent off for second-half lunges on Suat Serdar and saw their Bundesliga title hopes fade significantly.

But a famous victory inspired by two Daniel Caligiuri goals was marred by the appearance of a banner among the travelling supporters that read "Still a bombing [good] idea, freedom for Sergej W". 

Sergej W was the identifying name publicly given to the man convicted of 28 counts of attempted murder and sentenced to 14 years in prison after targeting Dortmund's squad with three explosions ahead of their April 2017 Champions League quarter-final match against Monaco. 

Retweeting a post reporting the Schalke banner, Real Betis centre-back Bartra – who was hospitalised by the attack and underwent surgery on a fractured wrist – strongly condemned those responsible.

"Intolerable, unacceptable and sad," he tweeted. 

"A football rivalry is one thing [but] it is another to demand the freedom of someone who played with the lives of 28 people." 

Schalke supporters could also face scrutiny after Dortmund winger Jadon Sancho appeared to be struck by an object thrown from the away end when celebrating Mario Gotze's 14th minute opener. 

The England international was off the field receiving treatment during the incident that led to Schalke's 18th-minute penalty – converted by Caligiuri after a VAR handball review controversially went against Julian Weigl. 

"When the penalty was awarded, Sancho received treatment on the sideline after being hit by a lighter from the crowd," Dortmund head coach Lucien Favre told Sky. 

"It really wasn't a good performance from the referee. Too much has happened today." 

Dortmund fans similarly faced criticism for their conduct after a large banner containing a homophobic slur was unveiled on the Yellow Wall terrace.