Dutch dressage rider Petra van de Sande criticised organisers after sandbags were placed around Greenwich Park's main arena today.
Van de Sande was furious that riders did not have a chance to familiarise themselves with the sandbags, propped against the dressage arena's perimeter fence because of the stormy weather, before their performances in the Paralympic Games Grade II team test.
Although she scored a respectable 68.095% and finished sixth on Valencia Z, Van de Sande could not be placated following a class that was frequently affected by heavy downpours.
"I was not happy at all. I am very angry because of the sandbags around the arena," she said.
"At the other Games that I've been to, when there were sandbags, the horses and riders were able to come out and get used to them before the actual performance.
"This time, it wasn't done, and I am really annoyed with the organisers."
Van de Sande's team-mate Gert Bolmer, who was drawn first to go, backed his colleague's stance.
"My horse (Vorman) was very good around the outside of the arena, but once we were inside she spooked several times," he said.
"I think she was scared of the sandbags. When we were training they weren't there, but they were there today."
LOCOG Paralympic dressage manager Amanda Bond said no formal complaints had been received after the decision was taken to deposit sandbags.
"We are aware of the comments, but a critical decision was made when the weather conditions turned stormy, and the sandbags were put there on safety grounds," she said.
"There have been no formal complaints, and it has been the same for everyone."
British rider Natasha Baker set a standard that no other rider in the 23-strong field could match.
Baker, who is riding as an individual only, scored 76.095% to set her up nicely for individual competitions on Saturday and next Monday.
Germany team rider Britta Napel went closest, posting 72.571% on Aquilina 3, while Canadian Lauren Barwick scored 72.095% with Off to Paris, but no other competitor topped 70%.
Rebecca Hart was the leading United States team rider on 69.095% with Lord Ludger, and she said: "I am very happy.
"It is the first international arena my horse has competed in, and we handled the rain very well. I think the rain put us into a bubble together and we used it as an aid."
Some riders found it tough going, though, with Italy's Antonella Cecilia retiring during her test and Australian Grace Bowman's horse Kirby Park Joy being spooked by the arena's giant scoreboard.